Peel the rhubarb, chop it into chunks, put it in a pan with some butter on a medium heat, add a little bit of sugar, cook until soft, stirring now and then. If you don’t want rhubarb, use damsons, apples, plums, blackberries, whatever kind of fruit or mix of fruit you want.
(That would be about 6 sticks of rhubarb, an ounce of butter (non-butter baking whatnot works fine) and a tablespoon of sugar.)
When it is stewed, melt 3 ounces of butter (or whatever, lactose free whatnot) in a saucepan. When just melted, turn off the hear and add 3 ounces of flour (or gluten free flour is fine) and 3 ounces of demarara sugar. No substitutions. Demarara sugar. Yes, it’s expensive, yes they don’t sell it everywhere in North America, yes you can tell the difference. You can substitute everything else in this recipe, but the demerara sugar is what makes it yummy. Rub it together with a wooden spoon. If it’s not like breadcrumbs, add more flour until it is. This scales easily, so you can do an extra ounce of everything if you’re making a big one, or as much as you need.
Put the fruit into a souffle dish. Sprinkle the crumble on the top. Bake in a hot oven (180 or 200) for 20 mins to half an hour. Reheats well. Is good cold.