1. Combine marinade ingredients and whisk well.
2. Pour marinade into a non-reactive dish or ziplock bag, and place the tuna steaks into the dish, turning to coat. Marinate for about one hour.
3. Grease a large grill pan with a bit of oil. On high heat, heat the pan until almost smoking. Grill tuna for one to two minutes per side and remove from pan. Set aside to rest, loosely covered with foil.