Grease the interior of the slow cooker crock with butter or nonstick cooking spray.
If you have time, cook the sausage in a skillet over medium, breaking up sausage with a wooden spoon. Stir in the onions after the sausage is no longer pink.Sauté the onions until soft.Use a slotted spoon to transfer the meat and onions to the prepared crock. Discard the drippings. Note: If you don’t have time, you can crumble the uncooked sausage straight into the crock. Stir in the onions, breaking up the meat with a wooden spoon.
Add the carrots, chicken broth, chopped stewed tomatoes, and tomato sauce, stirring to combine.
Cover. Cook on Low 4 to 5 hours.
Stir in the uncooked pasta and garlic.
Cover and continue cooking on Low 1 to 1 1⁄2 hours or until the pasta is tender but not mushy.
Stir in the mushrooms. Cover. Cook another 10 minutes on Low.
Just beforeserving,stir in the sliced spinach. (No need to cook the spinach; the heat from the soup will wilt it.)
To serve, divide the diced provolone or mozzarella cheese evenly into bowls and ladle the soup over the top.
Top with the Parmesan cheese and torn fresh basil.