Spread the diced onion and mushrooms evenly in the bottom of a 20 x 30cm baking dish. Arrange chicken pieces over the vegetables; skin side down. Squeeze lemon juice over the chicken pieces and season with garlic salt and pepper.
Bake chicken uncovered for 20 minutes in the preheated oven. Carefully turn the pieces over so skin side is up and continue cooking 15 minutes more.
Set oven to grill. Cook chicken for 5 minutes or until chicken skin is crisp.