Halve squash lengthwise; discard seeds. Fill with tomatoes and mushrooms; sprinkle with seasonings. Place in an oval 7-qt. slow cooker, tilting 1 slightly to fit.
Cook, covered, on low until squash is tender, 6-8 hours. Sprinkle with cheese. Cook, covered, on low until cheese is melted, 10-15 minutes. To serve, cut each half into 2 portions.