Spray a slow cooker container with olive oil-flavored cooking spray.
In prepared container, combine tomato soup, undrained tomatoes, water, and Italian seasoning.
Stir in uncooked pasta, onion, and chopped pepperoni.
Cover and cook on LOW for 6 to 8 hours.
Mix well before serving.
When serving, top each bowl with 1 tablespoon Parmesan cheese.