Preheat the oven: Set your oven to 350°F (175°C). Lightly butter a 9-inch baking dish or similar.

Prep the fruit: Peel, core, and slice the apples into thin wedges. Combine with blackberries in a large bowl.

Flavor and thicken: Add sugar, lemon juice, vanilla, cornstarch, cinnamon, and a pinch of salt. Toss gently until the fruit is evenly coated and glossy.

Make the crumble: In another bowl, mix flour, oats, brown sugar, granulated sugar, cinnamon, and salt. Add the cold, cubed butter.

Work in the butter: Use your fingers or a pastry cutter to rub the butter into the dry ingredients until the mixture looks like coarse crumbs with pea-sized bits. Stir in nuts if using.

Assemble: Tip the fruit mixture into the baking dish. Sprinkle the crumble evenly over the top, leaving a few small gaps for steam.

Bake: Place the dish on a baking sheet to catch any drips. Bake for 40–50 minutes, until the topping is golden and the fruit is bubbling at the edges.

Rest: Let it cool for at least 15 minutes before serving. This helps the juices thicken so each spoonful holds together.

Serve: Spoon into bowls and top with ice cream, cream, or custard. Enjoy warm.