1. Preheat oven to 190°C/375°F/Gas 5. Lightly grease an ovenproof pie dish, Pyrex bowl or casserole dish.
2. Put the rhubarb, golden caster sugar and water into a saucepan and simmer for about 10 minutes until rhubarb breaks down and softens.
3. Meanwhile, make the crumble by putting the flour, demerara sugar and oats into a bowl. Add the butter/margarine and rub into the dry ingredients.
4. Transfer the stewed rhubarb to the greased dish, spread the crumble mixture over this and bake for about 20 minutes until golden brown.
5. Serve with custard, freshly whipped cream or a dollop of Greek yoghurt.