Amanda Lydon spent childhood summers on Nantucket. Now running one of the island's best restaurants with her fiancé, chef Gabriel Frasca, she still finds pleasure in the island's food rituals.

It's the perfect day for a swim. Surfers line the horizon, taking advantage of a rare offshore wind that shapes the waves into long, perfect curls. But today, a group of us are kept ashore by other temptations: namely, a crate of live lobsters and littleneck clams. It is a sunny late-summer afternoon on a beach on the southern shore of Nantucket, Massachusetts. Some friends and I are here for a clambake hosted by my sister Amanda Lydon and her fiancé, Gabriel Frasca, chefs and managing partners of Straight Wharf Restaurant.

Several energetic guests dig a pit while Amanda and Gabriel carry rocks the size of human heads and more than four dozen hardwood logs from their truck. Friends and members of the Straight Wharf family, like waitress Chrystyna Kassaraba and bartender Kate Pelletier, fill shallow pans with the littlenecks and lobsters, potatoes, corn, chorizo and onions.

When the pit is two feet deep, the crew lines it with the rocks, then piles on the logs to start a bonfire. Once the wood has burned down to coals, the clam-filled pans are stacked in the pit between layers of rockweed, then covered snugly with three layers of damp canvas. It's an impressive feat that leaves everyone laughing and slightly breathless. Chrystyna's boyfriend passes out mason jars full of pale, icy Thai-basil sangria and tart tarragon lemonade. Amanda offers creamy smoked bluefish pâté on a tray with melba toasts, fresh sugar snap peas, radishes and cherry tomatoes as we wait for the clams to bake. The coals and rocks will create a smoldering oven in the sand, and we have some time to kill before that oven will cook the contents of the pans.

"That's the theory, anyway," says Gabriel, weighing down the tarps with rocks. "We haven't actually tried this before."

"But we have faith," Amanda interjects, with only the faintest hint of an eye roll.

Amanda and Gabriel met more than 10 years ago in the kitchen of a Cambridge, Massachusetts, bistro, Chez Henri. They have been together ever since, cooking side by side in Provence, France, and San Sebastián, Spain, and separately in some of Boston's most esteemed restaurants, including Truc (where Amanda was named an F&W Best New Chef 2000), Hamersley's Bistro, Radius and Spire. Through the same grinding restaurant schedules, they have shared a dedication to delicious, down-to-earth food. Last year they joined forces to take over the kitchen at Straight Wharf. That they have ended up running one of the most storied restaurants on Nantucket—where Amanda, our sister Honor and I spent our childhood summers—feels like fate.

Among the guests here this afternoon is Straight Wharf owner and founder Jock Gifford. As we dip into cups of fresh, delicate watermelon gazpacho spiked with sherry and jalapeños, Jock reminisces about when he and a group of talented friends decided to open the restaurant. They found the perfect space in an oddly situated seaside laundromat on the Straight Wharf, overlooking Nantucket's harbor. None of them had ever run a restaurant before; they preferred to think of Straight Wharf as an enormous dinner party. They opened on July 4, 1976. Under the leadership of chefs Susan Mayer and Marian Morash (author of The Victory Garden Cookbook), the food in those early days was simple and French-influenced—grilled locally caught fish, fresh salads and vegetables from island farms. The sense of elegant fun and camaraderie was contagious.

My family bought a house on Nantucket in 1973. We rarely ate out on the island, but as we were growing up, my sisters and I started earning pocket money by selling wild blueberries and blackberries to restaurants. Straight Wharf quickly became one of our best customers. We were shy but thrilled to greet Susan and Marian with our trays of just-picked berries. They exclaimed over our offerings as if the fruits were sapphires and rubies. One afternoon we were introduced to a tall, oddly patrician figure standing in the kitchen with the chefs; Julia Child cooed over our berries in those familiar, flutey tones we knew from her PBS broadcasts.

Our family associations with Nantucket have always had more to do with home cooking than restaurant food—summer dinners of steamed mussels and broiled bluefish served alongside tomato salads and black-berry pies. But I like to think that Amanda's interest in professional cooking was somehow kindled during those visits to Straight Wharf—by those brief, thrilling glimpses into what seemed like a happy kitchen, run by women and dedicated to a balance of high quality and island simplicity.

Indeed, when she was 19, during the summer of 1991, Amanda cooked at Straight Wharf. Twelve years later, when Gabriel spotted an ad the restaurant had placed for a chef de cuisine, Amanda encouraged him to apply. When he first described a midnight trip with a coworker to the jetties to fish for stripers—wading out into the black water with buckets of live baby eels slung around their necks—we suspected that he might be hooked on island life. In 2006, when Jock asked Gabriel and Amanda to take over the management of the restaurant, they leaped at the chance to helm an island institution, to bring their skill and passion to a place that they both knew and loved.

"We want to cook fantastic beach food," Gabriel explains. Nantucket appears in almost every dish, from the tart, wild Concord grapes in the grape soda float to the buttered toast made with local Portuguese bread that accompanies the halibut chowder. A simplified clambake is available in the dining room every night, and in the bar area, guests can dine on lobster rolls with plantain chips.

When word got out that Amanda and Gabriel would be running Straight Wharf, their staff seemed to materialize out of thin air. "We didn't have to put any feelers out—people just started calling us," Gabriel says. Scott Fraley, the former sommelier at Boston's Radius, joined them as partner and general manager. Other restaurant friends followed, eager to spend six months working in the seasonal island atmosphere. "We love that our guests are on vacation," Amanda explains. "Everyone's happier."

Straight Wharf in summer has a relaxed, open-air tempo that's hard to find in the city. Orders and checks at the restaurant are still written by hand, not on a computer. Amanda and Gabriel hired a local couple to plant a winding organic herb garden along the bar side of the building, and during summer lunches, guests frequently see kitchen staff picking basil, fennel fronds or nasturtiums. Just outside the kitchen windows is the harbor, with its boats and lights and frequent fog. Dinner service ends earlier here than in the city, and staff members often linger to chat and jig for squid off the dock before walking or biking home through downtown's cobbled streets.

Out on the beach, conditions are more rustic. A real clambake takes time. "This is why we always liked to do it in the kitchen!" Jock jokes, looking at his watch. At last the damp canvas tarp is peeled back, releasing a cloud of fragrant, salty smoke and steam. Plates are piled high with lobster, corn and clams, and the real eating begins. "Thank you, lobsters," someone murmurs gratefully, wielding a cracker.

Now in the late afternoon, it seems as if every surfer on the island has left work for the water. Gabriel passes around dessert—a plate of nutty zucchini cupcakes, bursting with bran-died raisins and topped with cream cheese frosting. We watch the surfers take ride after perfect ride and imagine the contentment of the wet-suited figures as they coast toward the beach. Up on the sand, as guests eat their cupcakes and sip the last of the sangria, the contentment is palpable, too.

Sarah Lydon lives in Boston. She has written for Saveur and Boston magazine.