Ivan Lyons knew the honeymoon was over. The first night home his bride, Nan, made tuna crunch. The next evening she roasted a chicken and left the bag of giblets inside. "Her cooking," he recalls, "was the pits."

After 20 years, many cookbooks, an arsenal of kitchen gadgets and hundreds of exotic culinary experiments, Nan's cooking has improved. "Tuna crunch is no longer one of my biggies," she says. All those hours over a hot stove yielded one additional confection, this one cooked up with her husband. In 1976 they published a frothy, urbane novel called Someone Is Killing the Great Chefs of Europe, which has been turned into a current movie. Renamed Who Is Killing the Great Chefs of Europe? (to avoid confusion with the new Farrah Fawcett-Majors film Somebody Killed Her Husband), the Lyonses' tale of terror among the toques blanches is a major hit. Their second book, Champagne Blues, about kidnapped travel writers in France, is due out next spring and has already earned $275,000 from Columbia Pictures.

Although, as Nan puts it, "money keeps appearing mysteriously in our mailbox," the Lyonses still maintain, Ivan says, "a broad streak of naiveté' Haggling over movie rights to Chefs in L.A.'s star-studded Polo Lounge was a fantasy come true. "We were like two Dorothys going down the yellow brick road," says Ivan, 44. Nan, 43, recalls, "I kept poking him in the knee and hissing, 'There's Peter Falk.' " Of the Warner Brothers version of Chefs, whose rights they sold with no strings attached, Ivan says only: "We're negotiating to write the screenplay for the next one ourselves."

Nan was a professional chorister who had never put anything on paper more ambitious than pompano en papillote. As president of a scientific consulting firm, Ivan's publishing experience was limited to his Information Hotline, a newsletter for the communications industry. (He, by the way, isn't allowed in the kitchen. "Everything he makes tastes like diesel oil," sniffs Nan. His response: "Lightning should strike your Cuisinart.")

Inspiration for The Great Chefs came on July 4, 1974. "We were sitting on a bench in Riverside Park," he recalls. "We wanted to do something exciting. Nan had an idea to write a mystery about a lady sleuth who cooks, and I was then dieting, so we conceived a murderer on a diet." Ivan wrote a chapter, then Nan collaborated on the rewrite. Chefs was finished by Labor Day and sold by December. "We weren't smart enough to have writer's block," says Nan. "We did nothing but laugh all summer."

Togetherness is important to the Lyonses. After they'd been married four years, Nan decided to quit the much-traveled Robert Shaw Chorale because "our phone bill was higher than my paycheck." In his previous job at a scientific publishing firm, Ivan found "work was an intrusion on the time we wanted to spend together." He started his consulting business and brought Nan in as bookkeeping assistant. Even there she found the job was too much billing and not enough cooing. Literary collaboration was the answer.

Book critics toasted their novel. It was roasted only when it fell into the hands of serious food writers, one of whom nitpicked that the oven used to dispatch a chef in London should have been preheated. Nan and Ivan have never even met any of the crème of New York's professional food critics. "It's just as well, really," she says, "although I'd love to meet James Beard. I make his English Muffin Bread every week."

The Lyonses were born 10 blocks apart in Manhattan, and both grew up lonely. He was an only child; she had one sister, 10 years older. Her mother sold dresses and her father was an organizer for the International Ladies' Garment Workers Union. "At one point we moved to Virginia," she recalls, "but we had to move back because my mother couldn't find a kosher butcher." After the High School of Music and Art, Nan, a talented mezzo-soprano, turned professional—but avoided the Met. Frankly, she finds opera "hilarious—I couldn't picture myself standing in front of a cigarette factory in a mantilla singing an aria." Instead, "I got an agent, and he fixed me up with choirs and choruses. I'd sing in synagogues, churches, anywhere. It was a very low-pressure job."

Ivan's father was a Russian immigrant pharmacist and, he says, "a bright Renaissance-man type. I was the only kid on the block with lots of books and the New York Times." He studied documentary film writing at City College, then took a fellowship at Boston University which he quit because "I missed New York." As a reporter for Show Business, a trade paper, "I was paid 25 cents an inch, wrote the paper from stem to stern and was always looking for more stories. One day I got this idea for an article about out-of-work singers, so I called up this terrible girl I'd met at a party."

"I thought he was dull, drab and snotty," Nan recalls, "but it was a free lunch. We had egg salad sandwiches at Whelan's." ("They were awful," Ivan recalls. "Too much celery.")

They got married a year and a half later, because, as he puts it, "I'd never met anyone as caring, sympathetic and funny." After a frugal two-day honeymoon in Newport, R.I., they set up housekeeping on Ivan's $60-a-week salary plus Nan's intermittent choral work. She eventually sharpened up her cooking by "reading, practicing and watching Julia Child." (She owns every conceivable gizmo for preparing food, but considers the television set "my most valuable piece of kitchen equipment.")

Meanwhile Ivan rose in the company like a soufflé, and as sales manager traveled to Europe a half-dozen times a year, taking Nan along whenever he could. "My business required me to stay in the best hotels," winks Ivan, who has made elaborate collages out of bills, letters, receipts and menus from some of them. Back home, they threw dinner parties for their "schoolteacher and stockbroker" friends. "I loved it," says Nan. "While other women were fighting to get out of the house, I was fighting to get back in." In 1969, after 11 years of marriage and "just being wonderful friends," the Lyonses decided to have a child. Samantha, now 8, "collects and clutters the same way we do, and already can tell a Bordeaux from a Burgundy," says Nan.

"We were really born at 40," Ivan says. "Everything has come to us late in life, but I think we enjoy it more now. We're a clear case of youth not being wasted on the young."

Their new joint career, which includes travel writing and a still-in-outline third novel, leaves Nan with little time to cook. "Often," Ivan sighs, "we just send out for pizza. But sometimes I get tough, in a respectful sort of way. I say: 'Hey, novelist, how about some breakfast?' "