Put the the sausage and chopped onion in a microwave safe bowl and microwave, stirring occasionally, until the sausage is no longer pink, 5 to 8 minutes.
Transfer the mixture to the slow cooker, breaking up any large pieces of sausage.
Stir in the bell pepper, mushrooms, garlic, broth, tomato sauce, tomatoes, salt and crushed red pepper. Cover and cook on LOW until the sausage and vegetables are tender, 6 to 8 hours.
Add the broken lasagna noodles, cover and cook on HIGH, until tender, about 30 minutes. Turn off the slow cooker and stir in the basil, mozzarella and Parmesan.
Ideal Slow Cooker Size: 4 to 5-Quart.
Can use a whole red bell pepper or whole green bell pepper.