1. Wash the potatoes well and cook in boiling water for about 30 minutes. Finely slice the spring onion and place in a large bowl with the capers, olives, olive oil and chopped parsley. Add a few spoonfuls of the potato cooking water.
2. Drain, peel and slice the potatoes. Add to the bowl and mix gently. Season with salt and pepper.
3. Drain and flake the tuna and add to the potatoes.
