Lightly season the fish and brush with a little oil. Leave to rest at room temperature while you make the relish. Heat the remaining oil in a deep frying pan and sauté the onions on a medium heat for 5 minutes, stirring occasionally. Once they begin to colour, add garlic and turn heat up to high. Stir continuously for a minute, then add lime zest and juice, sugar, tamarind paste and Thai fish sauce. Cook a further minute, then stir in coriander and basil leaves. Remove from heat.
Heat a griddle iron, skillet or heavy frying pan until really hot and smoking. Place tuna on and cook for only 45 seconds each side. The fish will be rare and moist inside. Remove to a plate and top with hot relish. Serve with crunchy steamed greens or a green salad.
© 2003 Peter Gordon. All rights reserved.