Arrange 4 large pieces of aluminum foil on a work surface.
Place 1 mahi mahi fillet onto each aluminum foil piece. Pour Italian dressing over each fillet. Sprinkle each fillet with lemon-pepper seasoning and garlic powder; flip fillets and season other side with lemon-pepper seasoning and garlic powder. Place 1 onion ring atop each fillet and squeeze lemon quarter over each fillet. Fold aluminum foil over each fillet, crimping all the corners to completely seal. Place packets in a glass baking dish in the refrigerator to marinate, about 2 hours.
Preheat oven to 325 degrees F (165 degrees C).
Bake in the preheated oven until fish flakes easily with a fork, about 30 minutes. Carefully open packets and remove onion before serving.