In a nonstick skillet saute' the pancetta until brown and crisp. Remove from pan and drain on paper towels; set aside.
Put the pepper, carrots, celery, onion, mushrooms and garlic in a food processor; pulse until finely chopped.
Over a medium flame, heat the olive oil and butter in a large, deep saute' pan with a thick, heavy bottom. To the pan add the chopped vegetables and lemon zest, saute' for 6 or 7 minutes - until the onions turn clear and the garlic starts to "dance". Pour all ingredients into a large bowl; reserve for later.
Continuing to cook over a medium flame, add the sausage links to the saute' pan, breaking into bite-sized clumps with a fork while it cooks; stir often. Cook until no longer pink. Remove sausage from pan and drain on paper towels. Meanwhile, discard all but 1 tablespoon of the pan drippings and add the beef to the saute' pan. Crumble with a fork to break the beef apart; stir often. Sprinkle with 1/2 teaspoon salt; cook until no longer pink.
Lower your flame to medium-low and return the sausage, sauteed vegetables and pancetta to the saute' pan; gently stir together to blend. Add the wine and simmer until the alcohol evaporates, about 3 minutes. Add the milk, allspice, basil, oregano and pepper. Simmer until milk thickens a little - around 3 or 4 minutes.
Stir in the tomatoes and a 1/2 cup of their juice. When the sauce starts to boil, reduce the heat so that it cooks at the barest simmer, with just an occasional bubble or two. Cook, uncovered, for 3 hours, turning down the heat if the sauce starts to scorch. If the sauce dries out before it is done, add a laddle of the reserved tomato juice or the beef broth; and check the seasoning.
The sauce will improve steadily as it cooks, and if you have the time simmer it longer - many Italian cooks suggest that it be simmered for 6 hours, adding the tomato juice or broth as necessary. When this bolognese sauce is done, it should be rich and thick.
Cook's Tip: The sauce can be stored in an airtight container and refrigerated for several days, or frozen for several months. Warm over low heat before serving.
Cook's Tip: Try using all chopped veal or lamb for a stronger flavor.