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 work surface. Reserve 2 teaspoons 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 from dish over fillets; garnish with lemon wedges, if desired.
The butter mixture gives the mild-tasting sole added flavor. Spread it down the center of each fillet, then put another spoonful on top of the folded fish.