To get a tender, flavorful steak, this recipe marinates top sirloin cuts in a mixture made with sherry, lime juice, and soy sauce, and then coats it in a spicy mustard before grilling so that it gets a nice, crisp crust to its juicy interior.
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.