Rinse the rhubarbs and chop them in 1/2 inch bits. Place them evenly in a cake pan or a spring form. If you use the latter, make sure you place some baking paper in between so rhubarb juice doesn't leak out.
Sprinkle the cinnamon evenly all over the rhubarbs. Then do the same with the sugar.
Melt the butter a bit in the microwave. Now mix it together with the flour, oats and sugar in a bowl. Mix it so it all blends well.
Now use your hands and sprinkle an even layer of the crumble on top of the layer of rhubarbs.
Put the pan in the oven and let it bake for 20-25 minutes. Take it out and serve with vanilla sauce.