1. Peel and cut the potatoes in slices. Place the slices in a pan of cold, salted water. Bring to the boil and cook for 20 minutes. Leave to cool. Drizzle with cider vinegar. Set aside at room temperature.
2. Combine the ingredients for the marinade. Cut the tuna into medium-sized cubes. Pour the marinade over the tuna, stir together, and keep in a cool place for 1 hour.
3. Cut the radishes into cubes and the cucumber into thin slices.
4. Place the potatoes, tuna, radishes, cucumber, and rocket in two large bowls. Pour over a little lime juice.
5. Serve garnished with sesame seeds and chives.

Tip: For an extra touch of colour and crunch, add small cubes of beetroot.