Prepare the spice mix: Blend the peppercorns, cloves, bay leaf, fennel seeds, red pepper flakes, and thyme in a spice grinder until finely ground. Set aside.
Cook the pancetta: In a large skillet or Dutch oven, cook the pancetta (or bacon) over medium heat until crispy. Transfer to a plate lined with paper towels.
Sauté the vegetables: Remove excess fat, leaving about 1 tbsp in the pan. Add onions, carrots, celery, and salt, stirring occasionally until golden brown (about 5-6 minutes). Stir in garlic and cook for another 2 minutes.
Slow cook the sauce: If using a slow cooker, transfer the vegetables to it. Add the spice mix, pancetta, ground beef, pork sausage, red wine, tomatoes, and tomato paste. Stir, breaking up the meat and tomatoes. Cook for 4 hours on high (or 3 hours on medium-low heat if using a stovetop Dutch oven).
Finish the sauce: Stir in milk and cook for 2 more hours on high (or 1 hour on medium heat for stovetop), stirring occasionally until thickened. Add red wine vinegar and adjust seasoning.
Cook the pasta: Bring a large pot of heavily salted water to a boil. Cook tagliatelle until al dente, reserving ½ cup of pasta water before draining.
Combine pasta and sauce: Toss pasta with butter in a large skillet over medium heat. Add bolognese sauce, mixing well. Stir in the reserved pasta water for a thick, glossy finish.
Serve: Top with burrata or Parmesan cheese, garnish with fresh basil, and enjoy!