Preheat and oil the grill. Bring the steaks to room temperature.
Place the soy sauce, sherry, mustard, lime juice, thyme, garlic salt, and pepper in a small bowl and whisk together.
Brush the steaks on both sides with the glaze. Grill over direct medium-high heat for 4 minutes. Turn the steaks over, brush again with the glaze, and grill for 4 minutes longer for medium-rare, 5 to 6 minutes for medium, or to the desired doneness. Remove from the grill and let rest a few minutes before serving.