Spread lentils into a shallow 2-quart baking dish. Pour water over lentils. Add tomatoes, carrots, green bell pepper, mushrooms, onion, celery, parsley, and garlic to lentil mixture. Cover dish with aluminum foil.
Bake in the preheated oven until lentils are tender, 1 1/2 to 2 hours. Remove aluminum foil and sprinkle Cheddar cheese over casserole; bake until cheese is melted, about 5 minutes more.
