Stir the sour cream and milk until smooth. Season to taste with salt, pepper and some lemon juice. Cut the cress from the bed. Stir 3/4 of the cress into the sauce
Wash and clean the zucchini. Clean and peel the fairy tails. Grate both roughly. Mix turnips, zucchini, flour and egg yolk, season with salt, pepper and nutmeg
Heat 1 tablespoon of oil in a coated frying pan. Add 3-4 tbsp. roasting mixture to the pan and fry for 3-5 minutes, turning over, until golden brown. Remove, keep warm and add the rest of the mixture and oil to make 12 Rösti.
Leek onions wash, clean and cut very diagonally into thin rings. Place the spring onions on a narrow side of the roast beef slices and roll up. Arrange the Rösti, rolls and sauce and sprinkle with the remaining cress.