Wash and scrub the potatoes and cook in their skins just covered with water for about 25 minutes.
Drain and flake the tuna.
Mix the tuna, yogurt, mayonnaise, lemon juice, salt, pepper and capers.
Check the seasoning.
Heat the oil and butter and briefly sauté the potatoes.
Add the lemon peel and parsley and season with salt and pepper.
Serve with the tuna dip.