Cut cold butter into cubes. Pour the dry ingredients (sugar, flour and oatmeal) into a container.
Gently combine with your fingertips (so that butter won’t melt in contact with the heat of tour skin) or with a food processor. See on the pics underneath the texture you should get. The idea is to break butter into small parts that will be coated with the other ingredients, not to have a very homogeneous paste.
Then arrange chosen fruits in a baking tin.
Cover with the crumble topping without pressing. Bake at 180°C for about 30 min.