Cut the rhubarb into pieces approx. 2 cm long, place on a baking sheet, sprinkle with the sugar and add 4 teaspoons of water. Bake at 190 ° C for 6 - 8 minutes. Carefully pour into a large sieve and allow to drain. Collect the juice.
Butter a baking dish and add the rhubarb pieces.
Mix the dough ingredients together and pour over the rhubarb.
Bake for 25 to 30 minutes at 190 ° C, the dough should tear off a little at the ends of the rhubarb. If the dough is still too soft, just bake for a few more minutes, up to a maximum of 40 minutes.
Take out of the oven and let rest for about 10 minutes. The dough will collapse a bit, this cannot be avoided and does not affect the good taste.
Before serving, brush with the collected juice and dust with powdered sugar.