Rinse and pat dry the tuna steaks and place in a zip lock bag.
Mix the remaining ingredients and pour over the tuna steaks, flipping the bag a few times to coat the tuna. Marinate in the refrigerator for 30 minutes -1 hour.
Spray the grill grate with the cooking spray. Turn the grill on to high and let heat for a few minutes. Turn the heat down to medium.
Gently remove the tuna from the zip lock bag. Brush the marinade on the tuna and then discard.
Place the tuna on the grill and cook for about 2-3 minutes per side (depending on how thick the steaks are). Tuna gets dry very quickly, so it is better to undercook it just a bit, leaving the inside pink.
Placing the fish under aluminum foil for a few minutes after removing from the grill will continue to cook the tuna.