Step 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).

Step 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.

Step 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.

Step 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.