Separate eggs. Beat egg whites until stiff. Cream egg yolk, 125 g sugar, vanillin sugar and grated zest of 1/2 lemon. Stir in quark. Mix semolina and baking powder and stir into the quark mixture.
Loosely fold in the beaten egg white. Grease the casserole dish (approx. 1 1/2 litres capacity) and sprinkle with 2/3 of the flaked almonds. Add the quark mixture, sprinkle with the remaining almonds and bake in the preheated oven (electric range: 200 °C/ gas: level 3) for approx. 30 minutes.
In the meantime, clean, wash and dry rhubarb and cut it into pieces of about 1 cm. Peel the remaining 1/2 lemon peel spirally. Boil up rhubarb, 1/4 litre water, lemon peel and cinnamon stick.
Whisk the starch in some cold water and bind the rhubarb with it. Bring to the boil briefly, remove from the heat and remove cinnamon and lemon peel. Season the compote with sugar if necessary and serve with the casserole.