First drain the tuna carefully over a small bowl or mug, reserving the oil. You’ll use this for cooking and for the dressing. If your tuna didn’t come in oil, don’t worry, just use a light cooking oil instead.
Drain and halve your potatoes and pop them in a large nonstick pan with a tablespoon of oil. Add the onion, and cook over medium heat for a few minutes, nudging with a wooden spoo