Add the onion and garlic to a food processor and blend until smooth and finely minced. In a large bowl, combine the ground beef with the onion mixture, eggs, breadcrumbs, fennel seeds, Parmigiano Reggiano, Italian seasoning, salt, and pepper.

Mix the meatball ingredients gently until fully combined, being careful not to overwork the meat. Shape the mixture into evenly sized meatballs and set them aside on a tray or plate.

Drizzle a little oil into the bottom of the crockpot and arrange the meatballs inside. Cover with the lid and cook on high for 2 hours until the meatballs begin to firm up.

While the meatballs cook, prepare the sauce by combining the tomato puree, crushed tomatoes, red wine, Italian seasoning, salt, and pepper. Slice the onions, mushrooms, and red bell pepper.

Pour the sauce mixture over the partially cooked meatballs and add the sliced vegetables on top. Cover the crockpot again and continue cooking on high for 2 hours and 45 minutes.

Once fully cooked, gently stir the meatballs and sauce together and adjust the seasoning if needed. Serve hot over pasta, polenta, or with crusty bread.