Rinse fish quickly under cold water. Pat dry with paper towels.Melt the butter in a small heavy skillet. Add the onion and sauté for about 3 minutes, stirring. Stir in the paprika and cook 1 minute more.Whisk in the mustard, lemon juice, salt and pepper and hot sauce. Remove from heat.Coat the fillets with the mixture and place in a shallow pan. Spoon over any remaining sauce.Broil under a hot broiler about 4” from the heat. Cook 3 to 4 minutes on the first side (depending on thickness). Turn once and finish for 2 to 3 minutes on the second side or just until the fish is cooked through.Place on serving plate and spoon over pan juices.Sprinkle with dill or parsley and serve.Serves 6.