Wash the potatoes thoroughly, cook in boiling water for about 20 minutes, drain, rinse and rinse off the skin. Let potatoes cool down. Meanwhile peel and finely dice onions. Bring stock and onions to the boil in a small pot and simmer for about 4 minutes. Stir in vinegar and horseradish. Season to taste with salt, pepper and sugar. Fold in the oil.
Cut the potatoes into slices, put them in a large bowl and pour the vinaigrette over them. Leave to stand for about 1 hour. Clean and wash the salad, pluck into bite-sized pieces and drain. Cut cress from the bed. Mix salad and 3/4 of the cress into the potatoes. Sprinkle with remaining cress on a plate and arrange with roast beef. Add the remaining roast beef
waiting time 1 1/4 hours