Preheat and prep the dish. Heat your oven to 350°F (175°C). Grease a 9-inch square baking dish or similar-sized casserole with a little butter or cooking spray.

Slice the apples. Peel, core, and thinly slice the apples so they cook evenly. Aim for slices about 1/4 inch thick.

Mix the fruit filling. In a large bowl, combine the apples, berries, granulated sugar, brown sugar, lemon juice, vanilla, cornstarch, cinnamon, and a pinch of salt. Toss until the fruit is evenly coated and glossy.

Transfer to the baking dish. Pour the fruit mixture into the prepared dish, spreading it into an even layer. If using frozen berries, add them straight from the freezer—no need to thaw.

Make the crumble topping. In another bowl, stir together the oats, flour, brown sugar, granulated sugar, cinnamon, nutmeg (if using), and salt.

Cut in the butter. Add the cold butter cubes to the dry mixture. Use a pastry cutter or your fingertips to rub the butter into the mixture until it forms pea-sized crumbs. Stir in the chopped nuts if you like.

Top the fruit. Sprinkle the crumble mixture evenly over the fruit. Don’t pack it down; loose crumbs get crispier.

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

Rest before serving. Let the crumble cool for at least 15 minutes. This helps the juices thicken and makes scooping easier.

Serve warm. Pair with a scoop of vanilla ice cream or a dollop of whipped cream for extra comfort.