In a large skillet over medium-high heat, cook the Italian sausage and ground beef together, breaking them up with a spoon, until browned and cooked through, about 10 minutes. Drain excess fat.
Transfer the cooked meat to your slow cooker.
Add the diced bell pepper, onion, garlic, cannellini beans, diced tomatoes, tomato sauce, chili powder, Italian seasoning, fennel seeds, salt, and red pepper flakes (if using) to the slow cooker.
Stir everything together until well combined.
Cover and cook on LOW for 5-6 hours or HIGH for 2½-3 hours.
Give it a good stir and taste for seasoning, adding more salt or spices if needed.
Serve hot with a sprinkle of Parmesan cheese and fresh basil, alongside crusty bread for dipping.