These shellsare sophisticated enough for any occasion. ServeParmesan cheese on the side.
Melt 3 tablespoons butter in a medium skillet over medium heat. Add leeks, and cook until soft, about 5 minutes. Add peas; cook until bright green, about 3 minutes more. Transfer mixture to a bowl, and season with salt and pepper. Stir in the crab-meat; cover with plastic wrap, and refrigerate until ready to assemble.
Melt remaining 6 tablespoons butter in a large saucepan over medium heat. When bubbling, add flour. Cook, whisking constantly, 1 minute. While whisking, slowly pour in milk. Continue cooking, whisking constantly, until mixture bubbles and thickens. Remove from heat; stir in cream and lemon juice, and season with salt and pepper.
Preheat oven to 375 degrees. Bring a large pot of water to a boil. Add salt and pasta shells; cook 2 to 3 minutes less than package instructions. Drain.
Meanwhile, in a small bowl, stir together breadcrumbs, garlic, parsley, and oil; set aside. Stir 1 cup cream sauce into reserved crab mixture. Pour another cup cream sauce into the bottom of a 9-by-13-inch baking dish.
Fill each pasta shell with a heaping tablespoon of crab mixture; arrange in baking dish. Spoon remaining sauce over shells, and sprinkle with remaining breadcrumb mixture. Transfer to oven; bake until mixture is bubbling and the top is golden brown, about 30 minutes. Let cool 5 minutes before serving.