For the groats, clean and wash the rhubarb and, depending on the thickness, cut the stalks in half lengthwise. Cut the sticks into pieces about 1 cm wide.
Mix 4 tablespoons apple juice and pudding powder until smooth. Bring 160 ml apple juice to the boil. Stir in the pudding powder. Stir in 100 g sugar. Add rhubarb, bring to the boil and simmer for about 5 minutes. If necessary add sugar and let the groats cool down.
For the zabaglione, wash the lemon hot, dry it and finely grate the rind. Mix the Marsala, egg yolk and 50 g sugar in a metal bowl with a whisk or mixer. Whip the egg mixture in a hot water bath for 7-10 minutes to a frothy cream.
Place the whipped zabaglione in a water bath with cold water and ice cubes and beat completely cold. Whip cream until stiff and fold in. Serve with rhubarb groats.