Set oven 400°F. Wash your fish, sprinkle garlic salt, pepper, put some fresh dill on greased baking dish/ tray, then place your fish skin down (if has it). Squeeze lemon all over the salmon. Slice lemon, and onions, and cover the fish with it Cover with aluminum paper, transfer into oven. Cook until fish gets flaky when you insert a fork, about 30 minutes. Take off the aluminum foil and turn on broil for about 10 mins, until onions and lemons turn their color

Prepare sauce: Mix chopped onions, horseradish, mayo and sour cream, and add dill. If you like, add few drops of lime juice. Serve with the fish.