Defrost the prawns and peas. Wash the potatoes and cook in boiling water for about 20 minutes. Drain potatoes, quench, peel and let cool. Clean, wash and cut spring onions into rings.
Bring the stock to the boil, add vinegar and 1 tablespoon of oil and stir in. Cut the potatoes into slices and pour the stock over them. Leave to stand. Rinse shrimps and pat dry. Heat 1 tablespoon of oil in a frying pan.
Sear the prawns briefly on both sides, season with salt and pepper and remove. Mix crème fraîche and horseradish, season to taste with salt and pepper. Add peas, spring onions and prawns to the potatoes, fold in crème fraîche, season again.