Preheat oven to 400 degrees. Coat 9x13-inch glass baking dish with cooking spray. Place chicken pieces in prepared baking dish. Sprinkle chicken with oregano, salt, and pepper. Place lemon slices evenly over chicken. Place onion slices over lemon slices, then tightly cover dish with aluminum foil. Bake 45 minutes, then uncover and bake an additional 10 to 15 minutes, or until juices run clear and no pink remains. Notes:For mixed citrus flavor make this with 1 lemon, 1 lime, and 1 orange instead of 3 lemons.