Preheat the oven to 180°C/160°C fan/ gas mark 4. Line a 23cm x 23cm square tin with baking parchment.
Put the butter and flour into a large bowl and use your fingers to rub together until the mixture resembles breadcrumbs. Add the sugar and rub again until combined, then stir through the oats. IF you squeeze the mixture, it should hold together.
Tip half the mixture into the bottom of the tin, pushing it right into the corners until it is a flat even layer. Spread the jam and grated apple over the base, then top with the chopped strawberries. Sprinkle over the remaining crumble topping and bake for 40 minutes, or until the topping is golden brown and the jam is bubbling through. Leave to cool for at least 15 minutes before slicing into squares. Eat warm or chill in the fridge later. These bars will keep for up to one week in an airtight container.