See my headnotes above about fish other than tuna that this will work for, as well as notes on exactly what sort of oil, lemon and salt are best for this recipe.
Slice the tuna thin, but not too thin. Shoot for about 1/4 inch thick. A tip on cutting cleanly: Draw your (very sharp) knife back towards you to cut, so you only slice in one direction. Sawing through fish makes it raggedy.
Arrange the fish on a plate or plates, and distribute the oil, lemon juice, capers, salt and lemon zest around it. Serve immediately,
Notes
A very dry white wine or dry rose are good choices here. This isn't really beer food, but a crisp pilsner would work.