For the potato salad: Cook the potatoes, and while still warm, add the capers and a good squeeze of the lemon. Add the oil, and toss in the red onion and celery heart and season carefully with salt and pepper.
Lay out your tuna steaks on a tray and season with salt and pepper. I prefer to cook it on one of those ridged grill pans or in a frying pan. Rub your pan, which should be very, very hot, with a little bit of olive oil on a piece of kitchen paper, then put in the tuna. What you want to do is sear the tuna so that it toasts, fries, and browns, about 45 to 60 seconds on each side. Once cooked, I like to rip the steaks in half and place them on top of the potato salad. Smash or pound up the basil in a mortar and pestle. Add a good glug of olive oil with a squeeze of lemon, and spoon this over each tuna steak.