2 medium onions, finely chopped
4 celery ribs, finely chopped
2 medium carrots, finely chopped
5 garlic cloves, thinly sliced
1/4 cup extra-virgin olive oil
1/4 lb pancetta or slab bacon, ground by butcher or thinly sliced and pulsed in food processor until finely chopped
1 lb ground veal
1 lb ground pork (not lean)
1 (6-oz) can tomato paste
1 cup whole milk
1 cup dry white wine
1 cup water
1 teaspoon fresh thyme leaves
1 1/4 teaspoons kosher salt
1/2 teaspoon black pepper
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 1/2 hours. Add salt and pepper and remove from heat.
Sauce may be made 2 days ahead and cooled, uncovered, before chilling, covered. Frozen, it keeps for 1 month.