In the 1980s, an assistant at Glamour took her romantic life to the next level with the aid of two lemons and a chicken. At the suggestion of one of the magazine’s editors, who was more or less following a recipe she’d found in an Italian cookbook, the assistant poked the lemons full of holes, stuffed them into the bird and loaded it into a hot oven. She ate the chicken with her boyfriend. Not long after, he proposed. Intrigued, other assistants tried the lemon-and-chicken trick on their own boyfriends. And lo, it came to pass that the halls of Condé Nast were soon glittering with the sparkle of new diamond rings.

The author of the cookbook was Marcella Hazan. If she had never done anything else in her life, Ms. Hazan would still have a guaranteed place in history as the progenitor of Engagement Chicken, a phenomenon so durable it has probably outlasted some of the marriages it was said to inspire.

Of course, Ms. Hazan did much more than that. She changed, thoroughly and irreversibly, the way Italian food is cooked, eaten and talked about in the United States. Although it has been 12 years since Ms. Hazan died, at age 89, and more than 30 since she put out a cookbook, nobody has yet overtaken her as the source Americans consult when they want to know how Italians get dinner on the table.

The new documentary “Marcella,” which opens at the Quad Cinema in Manhattan and begins streaming on May 9, ticks through a few of the things we can thank her for: Balsamic vinegar. Sun-dried tomatoes. The idea that there is no single “Italian cuisine” but many local ones, each with its own constellation of flavors.

I saw the movie in April at a screening at the Smithsonian Institution’s National Museum of American History. For the occasion, the curators unwrapped 19 cooking tools the museum acquired from Ms. Hazan’s kitchen last year. On display outside the theater were her square-cornered lasagna pan, her vintage garganelli comb adapted from a weaver’s loom, a linen apron printed with grapevines in dye made from vinegar and rust, and her wooden risotto spoon, which flares at the bottom like a rowing scull. (“You must never stop stirring,” she once wrote.)

Some of these items, along with the lined notebooks filled with recipes she wrote by hand in Italian that the museum also collected, are familiar from the photos, illustrations and endpapers of her cookbooks. One item not on view was her copper zabaglione pot, which the conservation department is getting ready to unveil next spring in an exhibition of 250 objects marking the 250th anniversary of the United States.

The Hazan trove isn’t as immense as that of Julia Child, whose kitchen has been rebuilt on the museum’s ground floor in all its cluttered glory, down to the paper-towel holder and plastic flip-top trash can. Looking at it, you can see how Ms. Child worked. Ms. Hazan’s artifacts show us something different. They are the products of her long campaign to bring the flavors of Italy to the country she adopted in 1955.

For many of the people who appear in “Marcella,” Ms. Hazan is more than a historical figure. She’s still with them.

“It sounds loopy, but Marcella’s voice is in my head as I’m cooking,” says Steven Sando, the bean merchant whose company, Rancho Gordo, sells a thin-skinned cannellini named in her honor. “And every time, she’s right.”

It doesn’t sound loopy to me. That voice — brusque, solidly accented, cured in cigarette smoke, marinated in Jack Daniels — comes to me all the time. Seeing cold pasta at a deli, I’ll hear her saying, “If I had invented pasta salads I would hide.”

When I can’t hear her, I freeze. I’ve stood for long minutes staring at boxed pasta in the supermarket trying desperately to remember which shape Ms. Hazan insists has to be used when you’re making Sicilian sardine sauce (bucatini or perciatelli). Fairly often, when the internet is drowning in a tidal wave of “Italian sushi” or some other mutant creation, I fantasize about hiring a medium to summon her spirit.

She delivered her dictums less as personal opinions than as natural laws. “The most useful thing one can know about basil is that the less it cooks, the better it is,” she wrote, as if this were a fact as ironclad as the tendency of water to flow downhill.

Although she claimed that she had never boiled water outside a laboratory before moving to New York to join her husband, Victor, she often sounded as if she learned to trim artichokes around the time of the Renaissance in a cooking academy taught by God.

Her overwhelming conviction that hers was the right way was daunting enough to the students of the cooking classes she began teaching in her Manhattan apartment in 1969. When she trained that confidence on the entire population of the United States, which included a fair number of Italians, the result was a small revolution.

Americans already thought they were in love with Italian food in 1973 when Ms. Hazan’s first book, “The Classic Italian Cookbook,” came out. What they were in love with was, in fact, the product of a mass migration of Italians who, more often than not, came from Campania, Sicily and other Southern regions. Many were fleeing the desperate rural poverty of tenant farms run under almost feudal conditions. Others were tradespeople with no formal education. Their marinaras, meatballs and lasagnas had evolved in their new country, but the roots were southern.

This world was not the Hazans’. Both Victor and Marcella were well-off northerners, from Romagna. Victor’s mother and father were Sephardic Jews who owned fur stores. When they left Italy, they were escaping, not poverty, but fascist antisemitism. Marcella’s parents were landowners whose tenant farmers paid a share of their earnings and brought them traditional tributes of chickens and rabbits when major holidays came around. Marcella was sent to universities in Padua and Ferrara, where she earned two doctorates in natural sciences.

Most of the recipes in “The Classic Italian Cookbook” were Northern Italian, too: roast lamb with juniper berries from Lombardy, Bolognese ragù with milk and nutmeg; minestrone in the style of Emilia-Romagna. She jotted them down in Italian, the only language she spoke when she moved to New York. Victor, her uncredited ghost writer, translated theminto English along with introductions stating his wife’s rigorous views on seasonality and simplicity. The style the couple hit on was stately, controlled, literary, erudite. It made allusions to Picasso and Aristotle. Above all, it was suffused with a belief that Italian cuisine was one of civilization’s great achievements.

“Nothing significant exists under Italy’s sun that is not touched by art,” that first book proclaims. “Its food is twice blessed because it is the product of two arts, the art of cooking and the art of eating.”

This was not the kind of message Americans were accustomed to hearing when they sat down to eat spaghetti by the flame of a candle stuck in a Chianti bottle while Dean Martin compared the moon in your eye to a big pizza pie. But by 1973, Italy’s image abroad had changed. It was now a beacon of style and the arts, the land of Fellini and Antonioni, Pucci and Valentino, Ferrari and Alfa Romeo. So when the Hazans came along selling the idea that Italy had also figured out a few things about good food that added up to a collective body of knowledge — in other words, culture — readers were ready to pay attention.

After her first cookbook, Ms. Hazan began collecting recipes around Italy, and she gave the food of the south its due. But she never warmed up to Italian American food, sniffing at its limp pasta, overcooked tomato sauces and heavy hand with garlic. Readers who were devoted to chicken scarpariello would come to see it and dishes like it as weird, bastardized aberrations. That is one of her legacies, too, for better or worse. When Carbone charges $94 for veal parm, some people seem to think it’s a scam. When Nello, a Northern Italian restaurant on the Upper East Side, charges $89 for veal Milanese, they just say it’s expensive.

It’s hard to imagine a recipe writer today changing the way a whole country thinks as thoroughly as Ms. Hazan did. Book contracts go to influencers whose advances are determined by their number of followers. Ms. Hazan didn’t have followers. She had disciples.

She still does. Peter Miller, who directed, produced and wrote “Marcella,” said almost all of the money for the film came from donations from hundreds of her fans.

“Everybody who gave money gave money because they love Marcella,” Mr. Miller said. “It’s not a sensible way to fund a film and it took a really long time, but I ended up building this whole network of people who knew her.”

The contributions were more than financial. Donors shared memories and photographs of Ms. Hazan that made their way into the documentary. One suggested Mr. Miller talk to Shola Olunloyo, a Nigerian-born chef in Philadelphia whose first non-African cookbook was one of Ms. Hazan’s. From it, he learned Bolognese her way, and has been making it about, once a week for two decades.

In another scene, the New York chef April Bloomfield cooks Ms. Hazan’s radically easy recipe for tomato sauce that bubbles away with butter and an onion that you fish out at the end, like a bay leaf. After tasting it, Ms. Bloomfield looks up toward the sky.

“Marcella, I hope you’re happy,” she says. “I hope I did a good job.”

If you own one of Ms. Hazan’s cookbooks, you know the feeling.