Preheat your oven to 190°C (375°F) / Gas Mark 5.

Prepare the fruit filling:
In a large bowl, combine the mixed berries, caster sugar, and lemon juice.
Gently toss the mixture to ensure all the berries are coated.
Spread the fruit mixture evenly in a large ovenproof dish.
Make the crumble topping:
Using your fingertips, rub the cold butter into the flour until the mixture resembles breadcrumbs. You can also use a pastry cutter or food processor for this step.
Stir in the rolled oats and caster sugar to the flour and butter mixture. Aim for a crumbly texture, with some larger pieces for a delightful contrast.
Assemble and bake:
Scatter the crumble topping evenly over the fruit filling in the baking dish.
Bake the crumble for 30-35 minutes or until the fruit filling is bubbling, and the crumble topping is golden brown.
Cooling and serving:
Let the crumble cool for at least 30 minutes before serving. This allows the filling to set slightly and prevents burning your mouth.