Preheat your oven to 180°C (350°F).
In a large saucepan over medium heat, combine the sliced apples, sliced pears, brown sugar, and cinnamon.
Cook, stirring occasionally, until the fruit is softened slightly, about 5 minutes.
Transfer the cooked fruit mixture into a baking dish, spreading it out evenly.
In another bowl, combine the plain flour, plain oats, brown sugar, and caster sugar for the crumble topping.
Add the cold cubed butter to the dry ingredients.
Using your fingertips, rub the butter into the dry mixture until it resembles coarse breadcrumbs.
If too dry, add milk a tbsp at a time.
Sprinkle the crumble topping evenly over the fruit in the baking dish.
Place in the preheated oven for 25 minutes, until the top is golden, and edges are bubbling.
Leave to cool for 10 minutes before serving.
Serve your Apple and Pear Crumble warm, either on its own or with a scoop of vanilla ice cream or a dollop of whipped cream.
