For the dough, coarsely grate the marzipan mixture. Knead with 200 g flour, 75 g icing sugar, 1 pinch of salt, egg and 100 g butter in pieces, first with the dough hooks of the mixer, then quickly with your hands to a smooth dough. Wrap the dough in foil and chill for about 1 hour.
For the compote, thaw the raspberries. Clean, wash and cut the rhubarb into pieces. Boil up with 1⁄4 l nectar and sugar and simmer for about 3 minutes. Remove rhubarb with a skimmer. Stir 100 ml nectar and pudding powder until smooth.
Stir into the boiling nectar, bring to the boil again and simmer for about 1 minute. Carefully stir in raspberries and rhubarb. Let the compote cool down and cover and let it cool down for about 30 minutes.
For the crumbles 75 g flour, oat flakes, 1 tbsp. icing sugar, egg yolk and 2 tbsp. butter first with the dough hooks of the mixer, then knead quickly with your hands.
Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Grease a tart mould with lifting base (26 cm Ø). Roll out the dough into a circle (approx. 30 cm Ø). Line the tin with the dough, press up the edge of the dough and cut straight.
Spread the compote on the dough, sprinkle the crumbles on the compote. Bake in a hot oven on the lowest shelf for about 40 minutes. Slide onto the middle shelf about 10 minutes before the end of the baking time. Let the tart cool down in the tin for about 10 minutes, then remove and dust with icing sugar. Best served lukewarm.