Cook spaghetti, drain. Combine egg, cream, milk, 2 tablespoons of parmesan, onion (salt and pepper) and mix. Add monterey jack cheese, mix well. Add spaghetti and spinach, pour into ungreased 10 x 6 baking dish. Sprinkle with remaining 2 tablespoons parmesan. Bake covered at 350 degrees for 15 minutes. Uncover, bake 15 to 20 minutes more, or until heated through.