Pre heat the oven to 220ºC. Use a a 20cm diameter pie dish which is about 1” deep. Place the berries/fruit in the pie dish.
Add the sugar and the corn starch over the berries and mix it carefully without mashing the berries.
Mix sugar, rolled oats and flour in a bowl. Add the the soft butter in smaller pieces into the bowl. Rub in the butter using your finger tips until it forms crumbs and starts to stick together.
Crumble the mixture loosely over the berries in the pie dish with your fingers, so that it leaves slight gaps and you can still see the berries underneath.
Bake it in the middle part of the oven for approximately 20 minutes until the crumbles get a crispy golden color.