Combine all ingredients except tuna in a 1 gallon zipper bag. Squish together until well combined, then add tuna steaks. Seal and let marinate for at least 30 minutes at room temperature, or if longer, in the refrigerator.
Preheat grill to 475. Grill approximately 3 minutes, turn, and cook an additional 2-3 minutes to desired doneness, basting while cooking.