Cut the rhubarb into thin slices and mix with the powdered sugar.
Mix together the spelled flour, almonds, baking powder and starch. Mix the buttermilk, sour cream, oil, eggs, sugar, syrup with the hand mixer on a low setting to a smooth mass. Briefly stir in the flour mixture in portions with the hand mixer on a low speed.
Spread the dough on a baking sheet lined with baking paper and fold in the rhubarb, also spread a few pieces on the dough.
Bake in the preheated oven at 170 ° convection on the second rack from the bottom for 25 - 35 minutes, depending on the oven, until golden brown.