Cook onions, celery, carrot, and garlic in oil in a 6- to 8-quart heavy pot over moderate heat, stirring occasionally, until softened, about 5 minutes.
Add pancetta, veal, and pork and cook over moderately high heat, stirring and breaking up lumps, until no longer pink, about 6 minutes.
Stir in tomato paste, milk, wine, water, and thyme and gently simmer, covered, until sauce is thickened, 1 to 1 ½ hours.
Add salt and pepper and remove from heat.
Cook noodles until al dente. Drain well, and add into sauce. Toss to coat.
Garnish with fresh parmesan cheese and a few thyme leaves.
Sauce may be made 2 days ahead and cooled, uncovered, before chilling, covered. Frozen, it keeps for 1 month.
