For the rhubarb crumble, butter 4 small ramekins. Squeeze the lemon. Wash the rhubarb and cut into pieces. Mix with 50 g cane sugar and lemon juice, pour into the ramekins, leaving the top edge free.
Mix oatmeal, the rest of the cane sugar, flour and cinnamon. Melt butter, add and mix until crumbs form.
Fill ramekins with crumbs and bake at least 20 minutes on middle rack at 180 °C. Let cool and serve the rhubarb crumble.