3. Start the Bolognese
Place a 5- to 7-quart Dutch oven over medium-high heat, add the butter, and let it melt. Once melted, add the pancetta and cook, stirring often, just until the pancetta starts to brown on the edges, about 3 minutes. Add the onion, celery, and carrots and cook until the vegetables are just tender and the onions are translucent, about 5 minutes.
Add the ground beef, ground pork, salt, and pepper. Stir to break up the meat and cook just until the meat is no longer pink, about 8 minutes. Add the pureed tomatoes, bay leaf, thyme, and nutmeg and stir to combine. Add the milk, white wine, and beef mixture. Adjust the heat to high and bring to a simmer.