Brush tuna steaks with oil and broil or grill them until cooked through, about 5 minutes on each side. Meanwhile heat the butter in a frying pan and slowly saute the garlic and onion until softened, about 5 minutes. Stir in the lemon juice and capers and cook for 1 minute. Spoon the sauce over the tuna and serve immediately. VARIATION: Tuna with Lime juice. Omit the capers. Substitute 1/4-1/3 cup freshly squeezed lime juice for the lemon juice and add 1/4 teaspoon good-quality soy sauce and a dash of cayenne pepper.