Heat a griddle pan until it gets really hot then put your asparagus in it. Place a heavy saucepan on top so that it presses the spears right down on to the bars. Cook them for 2 minutes, turning them halfway through, until they have lovely charred marks. Pop the asparagus on a board and chop them in half at an angle.
Put the asparagus in a bowl with your tomatoes, half the basil leaves, the olives, a splash of oil and a squeeze of lemon juice. Season, then toss it all together and put it to one side while you get the fish ready.
Rub your tuna steaks with a little bit of oil and season them. Sear them for about a minute on each side in the griddle pan for a rare steak – leave them for 2 minutes on each side if you prefer them cooked medium, or 3 to 4 minutes each side if you like them well done.
Pound the remaining basil in a pestle and mortar then mix in the yoghurt. Season and add some more lemon juice to taste.
Put the salad on a plate, then tear the tuna into strips and pop them on top. Spoon over some of the basil dressing and scatter over the baby basil leaves.