For the crumble: place the flour and oats in a mixing bowl, sprinkle in the baking powder and rub in the butter. When it all looks crumbly, and the fat has been evenly dispersed, mix in 75g sugar. The mixture is ready to sprinkle over your rhubarb. Cut the rhubarb into chunks and place in a saucepan with the remaining 75g sugar. Cook over a gentle heat, covered, for 5-10 minutes, stirring to get the uncooked pieces at the top down into the heat. It should be chunky not mushy. When the rhubarb is cooked, drain off about half the juice, then transfer it to a pie dish and sprinkle with the crumble topping. Bake for 30-40 minutes until topping is lightly browned.