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Put the butter in a small bowl and season with pepper. Finely chop the olives, capers and thyme leaves and stir into the butter. Chill until ready to use.
Heat a griddle pan or barbecue until smoking. Brush the steak with a little of the oil and lay on. Cook for 3-4 mins each side, then transfer to a board and cover with foil to rest.
Cut the ciabatta loaf in half, brush the cut side with remaining oil and place cut-side down on the griddle pan for 1-2 mins, until charred. Place the steak on the bottom half of the loaf and spoon the butter over it. Add the lettuce leaves and sit the top of the loaf on.