Preheat the oven to 350F (180C). With a handmixer, whip the sugar and butter together. Add the rest of the batter ingredients and mix until well combined. Transfer to a 20-cm springform and smooth the top with a spatula.
Add the rhubarb and press gently.
With your hands or the mixer, combine the crumble ingredients until the texture ressembles wet sand. Spread evenly over the rhubarb.
Bake in the preheated oven for around 50 minutes. If using wheat flour, the baking time might be shorter. The cake is ready when a toothpick inserted in the middle comes out clean.
To make oat flour, simply process oats in the blender until you obtain a fine powder.
This version baked in a 20-cm mould will stay quite juicy. If you prefer drier cakes, use a moul up to 26 cm big, but note that it will be quite thin since this flour mix won't rise as much as wheat or spelt).