Combine the tapenade (olive paste), oil, garlic, lemon rind and some black pepper. Spread over both sides of the tuna and refrigerate for 10 minutes.
Place the tuna on a lightly oiled, preheated BBQ plate (griddle) or grill and cook, turning once for about 3 minutes each side. When cooked, the steak should still be pink in the centre. Sprinkle with the spring onion.
Serving Suggestion: Baby spinach salad with toasted pinenuts.