Clean, wash and cut the rhubarb into pieces. Mix with 75 g sugar and lemon juice in a pot, bring to the boil and steam for about 2 minutes. Grease a box form (about 25 cm long). Spread rhubarb on the bottom of the tin and let it cool down.
Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). For the dough, whisk 125 g butter and 100 g sugar with the whisks of the mixer for about 5 minutes until creamy. Stir in the eggs one by one.
Mix flour and baking powder. Alternately stir in with the milk.
Spread the dough on the rhubarb and smooth it down. Grease the foil and cover the mould with it. Prick a few holes in the foil with a small knife or scissors. Place the mould in a fat pan (deep baking tray) and pour so much hot water on it that the fat pan is almost full to the brim.
Bake in a hot oven for approx. 1 1⁄4 hours (chopstick test).
Take out the casserole and remove the foil. Leave to cool for approx. 20 minutes, then use a knife to loosen from the edge and turn the casserole out of the mould. Eat warm and sprinkle with toasted almond flakes as desired
Vanilla sauce tastes good with it.