Easiest dish to make and ready in less than 5 minutes. Ensure the tuna steak has been brought back to room temperature (or thereabouts) if it has been in the fridge. Season both sides with salt and pepper. Depending on the thickness, just one minute or so per side on a grill (or in a pan) should be enough for a nice rare steak.
To increase tenderness, be sure to slice the tuna against the grain just like any other meat. Drizzle with sesame oil.