Gather all ingredients.
Cook sausage and onion in a large skillet over medium, breaking up clumps with a wooden spoon until browned; drain fat.
Add sausage mixture to a 4- to 5-quarts slow cooker. Add broth, tomato sauce, diced tomatoes, green pepper, Italian seasoning, garlic, and crushed red pepper.
Cover and cook on Low for 6 to 8 hours or on High 3 to 4 hours. If using Low, turn to High. Add pasta; cover and cook for 30 minutes.
Stir together ricotta, Parmesan cheese, and basil in a small bowl.
Top servings with ricotta mixture. Serve hot and enjoy!
