Heat a ridged griddle pan until hot. Split the rolls in half, add to pan and cook for 1–2 minutes on each side until lightly charred. Transfer to serving plates.
Brush the tuna steaks lightly with the oil and season with salt and pepper. Add to the pan and cook for 1 minute on each side.
Transfer each tuna steak to the base of a roll and squeeze over a little lemon juice from the lemon halves. Divide the tomato slices, pesto and salad leaves between the roll bases and drizzle over a little extra oil. Replace the roll tops and serve immediately.