Heat the oven: Preheat to 350°F (175°C). Grease a 9x9-inch baking dish or similar-sized pie dish.

Prep the apples: Peel, core, and slice the apples. Aim for evenly thin slices so they cook at the same rate.

Mix the filling: In a large bowl, combine apples with granulated sugar, brown sugar, lemon juice, cinnamon, nutmeg, vanilla, flour or cornstarch, and a pinch of salt. Toss until the slices are evenly coated and glossy.

Make the crumble: In a separate bowl, stir together flour, oats, brown sugar, cinnamon, and salt. Add the cold butter cubes. Use your fingers or a pastry cutter to rub the butter into the dry ingredients until the mixture forms coarse, sandy clumps. Mix in nuts if using.

Assemble: Spread the apple mixture evenly in the prepared dish. Sprinkle the crumble topping over the apples, covering them completely but without pressing down.

Bake: Place the dish on a baking sheet to catch drips. Bake for 40–50 minutes, until the topping is golden brown and the apple juices are bubbling around the edges.

Rest and serve: Let the crumble cool for at least 15 minutes to set the juices. Serve warm with a scoop of ice cream or a dollop of whipped cream.