Line a baking pan with foil.
Place salmon skin side down on foil. Sprinkle with lemon-pepper and onion salt. Top with onion and lemon. Dot with butter.
Fold foil around salmon; seal tightly, and bake at 350° for 20 minutes.
Open foil carefully, allowing steam to escape. Broil 4-6 inches from the heat for 8-12 minutes or until the fish flakes easily with a fork.