Combine ricotta, 1/2 mozarella, egg, spinach, mushrooms, and oregano.
Spread 1/3 of spaghetti sauce in bottom of 8x8 inch pan. Top with layer of noodles, 1/2 of spinach-cheese mixture, another layer of noodles, remaining half of spinach-cheese mixture, 1/3 of tomato sauce, another layer of noodles, remaining tomato sauce. Sprinkle with remaining mozzarella cheese.
Pour water into corners of pan. Bake about 1 hour, or until noodles are tender.