Brown 1 1/2 pounds ground beef with 1/2 cup chopped onion. Drain off fat. Stir in 1 (6 ounce) can tomato paste, 1 (2 ounce) can undrained mushrooms 1 to 1 1/2 teaspoon oregano, 1 1/2 teaspoon salt and 1/2 teaspoon pepper; simmer. Press refrigerator biscuits into greased 9 inch pie pan to form crust. Spoon meat mixture onto crust. Slice one tomato and arrange on top of pie. Sprinkle with 1 cup shredded Mozzarella or Cheddar cheese. Bake at 350 degrees for 20 to 25 minutes, or until golden brown. Let stand for 5 minutes before serving.