Thaw fish, if frozen.
Combine beer, oil, salt, garlic powder, pepper and mustard in shallow dish.
Marinate in refrigerator, covered, at least 30 minutes.
Remove fish from marinade; discard marinade.
Arrange fish on well-greased grid.
Grill, 4 inches from medium-hot coals, 8 to 10 minutes or until fish flakes easily when tested with fork, turning fish over halfway through cooking.
Melt butter in medium saucepan over medium-high heat.
Stir in paprika; add onions.
Saute until onions are tender but not brown.
Combine warm sour cream and horseradish in small bowl.
To serve, top each fish steak with onions and a spoonful of sour cream mixture.