Whizz together the flour & butter in a mixer (or if you're lazy like me and don't want any more washing up, chop the butter into the flour with a blunt knife) until it looks like breadcrumbs. (the first time I made this I rubbed it in with my hands and it turned into a lump of dough, so don't use your hands unless they are icy cold!)
Stir in the oats and sugar, mixing well. Tip it out over your dish of fruit, and bake at 180C/350F/Gas 4 until the topping is browned, oozy and stained by the juicy fruitiness underneath.