Preheat oven to 350 degrees F (175 degrees C). Line a baking sheet with aluminum foil; grease the foil lightly.

Place salmon fillet with skin side down on foil. Season salmon with lemon-pepper and onion salt; top with onion rings and lemon slices, dot with butter. Fold foil around salmon and seal tightly.

Bake in preheated oven for 20 minutes. Unseal the foil and turn the oven's broiler on. Continue cooking the salmon under the broiler until the flesh flakes easily with a fork, 8 to 12 minutes.

While the salmon bakes, stir sour cream, mayonnaise, white wine, capers, chopped onion, caper brine, lemon juice, horseradish, dill weed, garlic salt, and black pepper together in a bowl. Serve with the salmon.