Heat the oven to 400 degrees F. Cook and stir the meat in a large skillet until brown. Drain off the excess fat. Stir in half of the tomato sauce and the oregano leaves into the meat mixture. Heat to boiling then reduce the heat and simmer, uncovered, for 10 minutes.While the meat mixture is simmering, mix the baking mix, egg and the milk. Measure out 3/4 C. of the batter and set aside. Spread the remaining batter in a greased baking pan 9x9x2-inches. Pour into the remaining tomato sauce over the batter, spreading evenly. Layer 4 slices of the cheese, the meat mixture, the mushrooms and the remaining cheese on top of the batter and tomato sauce. Spoon the reserved batter on the top of the cheese. Sprinkle the batter top with the grated Parmesan cheese and bake, uncovered, until it is golden brown, 20 to 25 minutes. Cool for 5 minutes before cutting into squares and serving.