Combine the tuna steaks and lemon juice and marinate for 15 minutes. Prepare an outside grill with an oiled rack set 6 inches above the heat source. On a gas grill, set the heat to medium.
Grill or broil the tuna 6 inches from the heat source for 4 to 5 minutes on each side. Combine all remaining ingredients in a saucepan and bring to a boil. Pour the sauce over the tuna steaks to serve.