Heat some butter in a pan. Clean the leek, cut into finger-thick slices and add. When the leek is lightly browned, deglaze with a dash of water and let it simmer again (at this point I like to pluck the leek rings apart with a fork, then they cook faster).
Meanwhile, open the can of tuna, drain (or drink) the juice and add half of it to the leek. Now cut a quarter of the pepper into small pieces and transfer it to the pan as well. Season to taste with salt, pepper and other spices. The liquid has now almost evaporated and the dish is ready.