Prepare a charcoal or gas grill for direct heat.
Combine the first five ingredients in a glass dish or large resealable plastic bag. Add the steaks and turn to coat on both sides.
Marinate steaks in the refrigerator for at least 1 hour, but no more than eight hours.
Bring steaks to room temperature just before cooking. Sprinkle both sides with salt and pepper to taste.
Place steaks on a hot grill and cook for 5-6 minutes per side for rare (110 degrees). For medium-rare steaks, slide the steaks to one side of the grill away from direct heat and grill for an additional 7-8 minutes until the steak’s internal temperature reaches 120 degrees.
Let steaks rest for 10 minutes on a cutting board, loosely covered with foil.
Just before serving, drizzle with horseradish mustard sauce.
Combine all ingredients in a small bowl. Season with additional salt and pepper if desired.
Store any leftovers in an airtight container. Sauce will keep up to a week.