The setting was a buttercup-sprinkled mountain meadow straight out of The Sound of Music. The guests, headed by Jack Hemingway and his wife, Puck, were la crème from Ketchum and Sun Valley, Idaho. And the menu—tacked to an aspen tree—provided the most convincing evidence that as picnics go, this was one in a million, not run-of-the-mill. Among other delights were strawberry daiquiris, chicken roasted and stuffed with wild garlic and herbs, fait de concombre à la men the (yogurt, cucumbers and mint) and—for a touch of pure Americana—four-bean salad.

Masterminding the repast was 25-year-old Joan Hemingway (Muffet to her family and friends), who should know all about elegant eating in the boonies. She is co-author (along with her pal, Connie Maricich, a nearby Sun Valley boutique owner) of a book on the subject, tentatively titled The Picnic Gourmet, to be finished this fall. The book will receive more than perfunctory attention, partly because Muffet is the daughter of Ernest Hemingway's eldest son, Jack, the sister of newly married "million-dollar model" Margaux Hemingway (PEOPLE, Dec. 23) and the co-author herself of Rosebud, the thriller novel which Otto Preminger made into a movie last year.

Muffet's prescription for a picnic is very special. "You hike all day, getting absolutely tired. Then you reach a mountain lake, set the table, decorate it with wild flowers, and cool the wine in the lake." How the al fresco meal is served is as important as the choice of a good wine. "Cloth napkins and a pretty tablecloth," insists Muffet, who also on special occasions goes for such nonrustic touches as floppy cushions, enameled Chinese plates and big wine goblets.

The picnics began as part of a local ritual of hiking into the hills each Wednesday. The sophistication to which the picnics have evolved can be a shock to the uninitiated. Recalls filmmaker Roger Sherman: "I thought I knew what food was like on a hike, so I brought a couple of oranges. When we stopped to eat, out came a blue groundcloth, and then the most incredible food—asparagus wrapped in ham, cold watercress soup, whole-wheat bread baked by Muffet."

"Our hikes have gotten more decadent," admits Sun Valley shopowner Millie Wiggins. "We used to walk longer and carry less food. Now we take shorter hikes and bring more food." Sometimes the hike is dispensed with almost entirely. Men in cowboy duds and women in long gowns bring along dishes that have been commissioned by Muffet, and the only exertions, all optional, are frisbee-tossing, dancing or wading in the icy Corral Creek.

Muffet, who studied at Paris's Cordon Bleu and picked up other points from her mother, has mastered everything from moussaka to prune bread. She admits she wasn't always so enamored of the bucolic life. She was raised in San Francisco—and frequented the drug-heavy Haight-Ashbury scene—before her family moved to Ketchum eight years ago.

Now Muffet loves the mountains as much as her grandfather Ernest, who first introduced the Hemingways to the Sun Valley area in 1939. Ideally, Muffet would like to spend the picnic months in Idaho and winters in Paris, where she works on documentaries and feature films. She is anxious to dispel rumors of friction between her younger sister Margaux, 20, and herself. "I think there is competition," Muffet says. "I want to be just as good and on top as she is, but in other fields. As far as envy or jealousy goes, we transcended that as little kids."