Sauce:
Combine half the mushrooms, the onions and garlic. Add water, tomato paste, basil, oregano, and red pepper. Bring to a boil, reduce heat. Simmer uncovered for 4 minutes, stir occasionally. Remove from heat.
Meanwhile, spray 4 shallow individual casseroles with nonstick coating. Measure thickness of fish. Cut fish into equal portions. Place fish in casseroles, tucking under any thin edges. Pour sauce over fish. Top with remaining mushrooms, green pepper and cheese. Bake in 450 degree oven until fish flakes easily when tested with a fork (allow 5 to 7 minutes per 1/2 inch thickness).