DIRECTIONS
In a small bowl, stir together butter, parsley, lemon zest and juice, horseradish, and mustard.
Season both sides of fillets with salt; lay flat side up on a clean surface.
Reserve 2 tsp. butter mixture.
Dividing evenly, spread remaining butter mixture on top of fillets.
Fold fillets in half crosswise enclosing butter mixture.
Place folded fillets in a 9-inch microwave-safe dish with a tight fitting lid.
Top with reserved butter mixture, dividing evenly.
Cover; microwave on high until fish is just cooked through, about 5 minutes.
To serve, place fillets on plates and spoon juices over fillets.
Serve 4.