We have some rhubarb that grows in our backyard, and every year we have a good harvest. We sell some by the side of the road, give it away to friends, and still have enough left over to bake with. I put this crumble together to take to a game night. It was well received. Here's the basics of what went into this one.
I started with maybe a half cup of white sugar, a couple tablespoons of flour, and a pinch of cinnamon. Your going to combine this with the rhubarb, so the flour is to absorb some of the liquid when the rhubarb cooks down. The amounts are a bit variable, depending on how juicy your rhubarb is, and how sweet or tart you want the final product to be.
Combine with about 6 cups of chopped and cleaned rhubarb. It's very important that no leaves are used in cooking. Just that stalks. In retrospect, I should have added a bit more flour, because my Rhubarb mixture ended up pretty wet. Into a baking dish it goes.
Next up you make the topping. This is equal parts brown sugar and oats, plus another pinch of cinnamon, pinch of nutmeg, and about a half stick of butter, cut into small chunks. Combine these together. You'll want to use a pastry cutter or a fork because it's going to stick. The idea is to cover the chunks of butter with the dry mixture. Sprinkle this on top of the rhubarb mixture.
Then pop it into a 375 F oven until the top is just a bit golden. Check it every 5 minutes starting at 25 minutes.
Mine came out really well, but was a bit wet. Like I said, I should have added more flour in with the rhubarb. It ended up having a pudding consistency. Pudding in the British sense, not the Snack Pack kind.