Put the flour in a bowl. Press a depression in the middle and crumble the yeast into it. Sprinkle a teaspoon of sugar over the yeast. Warm the milk lukewarm. Pour 1/3 of the milk over the yeast and mix with the yeast and some flour.
Cover the pre-dough and let it rise in a warm place for about 15 minutes. Melt the fat in the remaining milk. Add egg, remaining sugar and salt and whisk everything. Add the liquid to the yeast pre-dough and mix everything to a smooth dough with the dough hooks of the hand mixer.
Let the yeast dough rise for another 20-30 minutes. In the meantime clean, wash and dry rhubarb and cut it into pieces of about 2 cm. For the crumbles, put flour and sugar on a work surface.
Spread fat in flakes on top of it. Chop through everything with a large knife and then knead into crumbles with your hands. Grease a baking tray. Knead the yeast dough again, roll out evenly on the baking tray and prick several times with a fork.
Place rhubarb pieces on the dough and spread the crumbles on top. Bake in the preheated oven (electric: 200 °C/ gas: level 3) for 40-45 minutes. Results in about 24 pieces.