Instructions:
- Brown the ground Italian sausage in a large skillet over medium-high heat, breaking it up with a spoon. Drain off most of the rendered fat and set the meat aside.
- Add a splash of olive oil to the skillet if necessary. Add chopped onion and fennel. Sauté until softened, about 5-7 minutes. Add garlic, oregano, basil, and red pepper flakes; cook for 1 minute until fragrant.
- If using wine, pour it into the skillet and scrape up any browned bits (fond). Let it simmer and reduce by half (about 2 minutes).
- Transfer the cooked sausage, the sautéed vegetable mixture, crushed tomatoes, tomato paste, Worcestershire sauce, and brown sugar into the slow cooker insert. Stir well to combine thoroughly.
- Season generously with salt and freshly ground black pepper. Cover the slow cooker and cook on LOW for 6 to 8 hours or HIGH for 2 to 3 hours.
- About 30 minutes before serving, taste and adjust seasonings. If the sauce is too liquid, remove the lid and cook on HIGH for the last 30 minutes to reduce slightly.
- Stir in about half of the shredded smoked mozzarella until melted and incorporated into the filling.
- Spoon the hot filling onto lightly toasted buns. Top each sandwich with the remaining shredded mozzarella and a sprinkle of fresh parsley. Serve immediately.