Prepare the Vegetables: Heat the olive oil in a large skillet over medium-high heat. Add the carrots, celery, and onion, and cook until they begin to soften, about 5–7 minutes.
Add Zucchini: Stir in the diced zucchini and cook for another 3–4 minutes, until it starts to soften.
Cook the Meat: Push the vegetables to one side of the skillet and add the ground beef and pork. Cook, breaking up the meat with a spoon, until browned.
Add Garlic: Stir in the minced garlic and cook for an additional 1 minute until fragrant.
Deglaze with Wine: Pour in the red wine and reduce the heat to low. Simmer until most of the wine has evaporated.
Combine Ingredients in Slow Cooker: Transfer the mixture to a slow cooker. Add the crushed tomatoes, beef broth, tomato paste, oregano, basil, thyme, salt, pepper, and bay leaf.
Cook Slowly: Cover and cook on low for 6–8 hours, stirring once or twice.
Finish the Sauce: About 30 minutes before serving, stir in the whole milk and continue cooking. Just before serving, stir in the fresh parsley and remove the bay leaf.
