Cheese Stuffed Shells with Bolognese Sauce

In a heavy bottom pot over medium heat, cook the onion, celery, carrot, and garlic in the olive oil. Cook stirring occasionally for 5-7 minutes until vegetables are softened and season with salt and black pepper. Add the pancetta, veal and pork and cook over high heat, stirring to break up the lumps. Cook until the meat is fully cooked (about 6-8 minutes.) Stir in the tomato paste, milk, wine, water, and thyme and gently simmer covered until sauce begins to thicken (about 1 hour.)  Adjust seasoning with salt and black pepper. Preheat oven to 350°F. In a casserole dish, cover the bottom of the baking dish with some of the Bolognese sauce. Add the stuffed shells to the baking dish and cover the top of the stuffed shells with additional Bolognese sauce. Cover the baking dish with foil and bake for 30-35 minutes. Divide among serving plates and sprinkle with Parmesan cheese if desired. Serve immediately.