Line a 15x10x1-in. baking pan with heavy-duty foil; grease lightly. 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,, Bake at 350° for 20 minutes. Open foil carefully, allowing steam to escape. Broil 4-6 in. from the heat for 3-5 minutes or until the fish flakes easily with a fork., Meanwhile, combine the sauce ingredients until smooth. Serve with salmon.