Wash the potatoes thoroughly, scrubbing the skin with a vegetable brush if necessary. Cover and cook in water for about 20 minutes.
In the meantime, wash the cucumber, peel if necessary and grate coarsely. Peel and finely chop the onion. Wash tarragon, remove leaves. Stir curd and mineral water until smooth. Stir in cucumber, onion, tarragon and lemon peel.
Season to taste with salt and pepper.
Drain the potatoes, let them steam briefly, then halve and possibly peel them. Sprinkle sea salt over the potatoes. Serve with curd cheese and prawns.