Combine 1/2 cup dry sherry, 2 tbsp gingerroot, grated, 2 tbsp soy sauce, 1 tbsp molasses, 1 tbsp lime juice, 1 tbsp honey, 1 tbsp garlic, minced, 2 tsp dark sesame oil, and 2 tsp cilantro, minced in a small bowl. Set aside. Rinse tuna steaks and pat dry. Place steaks in a shallow glass baking dish and pour sauce over. Marinate in refrigerator for 10 minutes. Preheat stove top grill or gas grill. Remove steaks from marinade; grill 5 to 8 minutes on each side for well-done center (2 minutes on each side for rare center and 3 to 5 minutes each side for medium center), turning once and basting with marinade.