1. Heat butter and oil in a large pot over medium-high heat until butter starts to sizzle, then reduce heat to medium. Add pancetta, and cook until golden and fat has rendered, about 2 1/2 minutes. Add onion, carrot, and celery, and cook, stirring often, until just beginning to brown around edges, about 10 minutes (adjust heat if mixture is browning too quickly).

2. Add beef and pork and cook over medium heat, stirring frequently and separating meat with the back of a wooden spoon, until no longer pink, 8 to 10 minutes. Once meat is completely browned, pour off any excess fat. Add tomato paste and cook 1 minute, stirring to intensify sweetness.

3. Pour in wine and cook, stirring to scrape up browned bits from bottom of pot, until liquid has evaporated, 6 to 7 minutes. Add 1 cup milk and cook until reduced by half, about 3 minutes (don't worry if it appears slightly curdled, it will smooth out again). Add thyme bundle and bay leaves, and then pour in tomatoes and 6 cups stock. Season with 1 1/2 teaspoons salt and 1/4 teaspoon pepper.

4. Bring to a boil, then reduce heat to a very low simmer and cook, partially covered, 3 to 3 1/2 hours, skimming the fat from the surface with a ladle periodically. If at any time the sauce appears too dry, add up to 1 cup more stock as necessary. The finished sauce should have the consistency of a loose chili. Stir in remaining 1/4 cup milk and season with salt and pepper, as desired. If not serving immediately, let cool completely before transferring to airtight containers. Refrigerate up to 3 days or freeze up to 3 months; defrost in the refrigerator before using.