Found this delicious recipe in Cuisine at Home. The bolognese alone sounds wonderful and the ricotta-stuffed shells are a bonus. As with all homemade sauces, this recipe takes a little more time, but is well worth it.
Preheat oven to 400 degrees F. Bring a large pot of salted water to a boil for the shells. Brown sirloin for the sauce in oil in a large saute pan over medium-high heat. Spoon off fat, add onion, celery, and carrot, and cook until soft, 5 minutes. Stir in tomato paste and garlic; cook 2 minutes, then add wine.
Simmer until wine is reduced by half, then stir in tomatoes, milk, salt, basil, oregano, and pepper flakes. Cook 15 minutes, stirring occasionally; finish with mascarpone. Spoon 1/4 cup sauce into each of 8 individual ovenproof casserole dishes (or spread 2 cups sauce in one 3-qt. baking dish). Set dishes aside.
Cook shells just until pliable, about 8 minutes (they will finish cooking in the oven). Drain, then plunge into ice water to stop the cooking; drain again and set aside.
Combine ricotta , part-skin mozzarella, parmesan, parsley, eggs, and salt in a large bowl; stuff mixture into shells. Arrange shells in the prepared dishes on top of sauce, cut side down. Spoon remaining sauce over shells, top with fresh mozarella, and bake until cheese is melted and sauce is bubbly, 25-30 minutes. Let shells rest 10 minutes before serving.