Mix the sugar and the flour together in a bowl and rub the butter into it, so you are left with a crumbly mixture. Add the almond slivers and the zest and your crumble is done.
Cake
Cut the ends of the rhubarb stalks and cut them into 2 cm pieces. You need a total of 325 grams of rhubarb cleaned.
Preheat the oven to 180˚C (350 F). Place the icing sugar, flour and cinnamon in a sieve and shake into a bowl. Add the ground hazelnuts and the almond flour and mix together.
Whisk your egg whites to soft peaks
Spoon the butter and the eggwhites into the flour mixture and fold it through. Pour the batter into your baking tin and divide half of the crumble mixture over it. Add your rhubarb pieces and than end with the other half of the crumble.
Sprinkle the dark brow sugar over the top and bake the cake for about 40 minutes in the oven or until firm and cooked.
Leave to cool for ten minutes in the tin. Serve hot or at room temperature.