Cut off stem end of each bell pepper (about 1/2 inch); remove and discard seeds and membranes. In large bowl, mix rice, beans, mushrooms, prechopped vegetables, thyme, salt, pepper and 1/2 cup of the cheese. Spoon mixture evenly into bell peppers.
Spray 6-quart slow cooker with cooking spray. Spread pasta sauce in slow cooker. Arrange peppers over sauce. Sprinkle with remaining 1/2 cup cheese.
Cover; cook on Low heat setting 4 hours or until peppers are tender.