For the sorbet, wash and clean the rhubarb. Cut 250 g rhubarb into pieces. Boil up with 250 g sugar and 300 ml water and simmer for about 10 minutes. Remove from the heat and let it rest for about 20 minutes.
Pass the rhubarb through a sieve. Mix rhubarb with lemon juice. Beat the egg whites until stiff, fold into the lukewarm puree. Freeze the sorbet mixture in an ice-cream maker or put it in a flat mould and freeze for 3-4 hours, stirring several times.
For the compote, cut 500 g rhubarb diagonally into pieces. Put it into a pot, sprinkle with 50 g sugar and let it stand for about 15 minutes. Then bring to the boil with rhubarb nectar and vanilla sugar and simmer covered for about 2 minutes.
Mix the starch with 3 tbsp. water until smooth, stir into the compote and simmer for about 1 minute, stirring continuously. Let it cool down.
For the quark cream, mix quark with 1 tablespoon of sugar. Divide into four bowls, sprinkle with 2 tablespoons of sugar each. Cut off 4 balls from half of the sorbet, freeze the rest of the sorbet again (it will keep for about 3 months in the freezer).
Serve the quark with compote and sorbet balls. Scald the curd with a kitchen gas burner.