Heat gas grill on medium or charcoal grill until coals are ash white.
Press cracked pepper onto both sides of steaks. Place steaks onto grill. Grill, turning once, until internal temperature reaches 145°F (medium-rare) or until desired doneness (15 to 20 minutes).
Meanwhile, combine beef broth, garlic and sherry in small saucepan. Cook over medium heat until mixture comes to a boil (5 to 7 minutes). Continue boiling for 5 minutes. Stir in sour cream and mustard with wire whisk. Continue cooking until heated through (1 minute).
Serve sauce over grilled steaks. Sprinkle with additional cracked pepper, if desired.