1. Combine mustard, honey, horseradish, mint, 1/4 teaspoon salt, and 1/4 teaspoon pepper in a small bowl; whisk well. Let mustard mixture sit at room temperature to allow flavors to meld (about 15 minutes).
2. Combine remaining salt and pepper in a shallow dish; press both sides of steaks into mixture. Heat oil in a nonstick pan over high heat until oil begins to shimmer. Place steak in pan; cook until a crust has formed (about 4-5 minutes). Turn steaks over; continue cooking until desired degree of doneness (about 3 minutes for rare, 4 minutes for medium-rare, and 5 minutes for medium). Remove steaks from pan; let rest 5 minutes before serving. Serve steaks with mustard sauce.