Step 1: In a large skillet or Dutch oven, heat olive oil over medium heat. Add the diced pancetta and cook until crispy. Remove pancetta with a slotted spoon and set aside, leaving the rendered fat in the pan.
Step 2: Add the ground beef and Italian sausage to the pan. Break up the meat with a spoon and brown it thoroughly on all sides, about 8-10 minutes. Drain off any excess fat, leaving about 1-2 tablespoons.
Step 3: Add the diced onion, carrots, and celery to the pan. Sauté over medium heat until softened and slightly caramelized, about 8-10 minutes. Add the minced garlic and cook for another minute until fragrant.
Step 4: Pour in the red wine, scraping up any browned bits from the bottom of the pan with a wooden spoon. Let the wine simmer and reduce by half, about 3-5 minutes.
Step 5: Transfer the meat and vegetable mixture to your slow cooker. Stir in the crushed tomatoes, tomato paste, beef broth, dried oregano, dried basil, bay leaves, salt, and black pepper. Stir well to combine.
Step 6: Cover the slow cooker and cook on low for 6-8 hours or on high for 3-4 hours, until the sauce is rich, thick, and the meat is very tender. Remove the bay leaves before proceeding.
Step 7: About 30 minutes before serving, stir in the milk or heavy cream. Continue cooking until heated through and the sauce is creamy. Serve hot over your favorite cooked pasta, garnished with fresh Parmesan cheese and parsley if desired.