Classic Ragù Bolognese

Name: Classic Ragù Bolognese
Ingredients: 2 Tbsp. extra-virgin olive oil; 2 medium onions, finely chopped (about 1 1/2 cups); 2 celery stalks, finely chopped (about 1 cup); 2 carrots, peeled, finely chopped (about 3/4 cup); 6 oz. ground beef (85% lean); 6 oz. ground veal; 3 oz. thinly sliced pancetta, finely chopped; 1/2 cup dry red wine; 3 cups (about) beef stock or chicken stock, divided; 3 Tbsp. tomato paste; Kosher salt and freshly ground black pepper; 1 cup whole milk; 1 lb. tagliatelle or fettuccine (preferably fresh egg); Finely grated Parmesan (for serving)