Sprinkle fillets evenly with 1/2 teaspoon salt; press cracked pepper into all sides of fillets.
Melt 1/4 cup butter in a large heavy skillet over medium-high heat; add beef, and cook for 3 to 4 minutes on each side or until beef is desired degree of doneness.
Melt remaining 1/4 cup butter in a saucepan over medium heat. Whisk in flour, remaining 1/4 teaspoon salt, and ground pepper until blended; cook, whisking constantly, 1 minute. Add cream, and cook, whisking constantly, 1 minute or until thickened and bubbly. Stir in horseradish and mustard. Drizzle over fillets.