Preheat and prep: Preheat your oven to 350°F (175°C). Grease a 9x9-inch baking dish with a small amount of butter or nonstick spray.

Prepare the apples: In a large bowl, toss the sliced apples with sugar, cinnamon, nutmeg, and lemon juice. Make sure every piece is coated lightly; this ensures the filling will be flavorful and aromatic.

Make the crumble topping: In a separate bowl, combine flour, oats, brown sugar, cinnamon, and salt. Add the cold diced butter and use your fingers or a pastry cutter to mix until the mixture resembles coarse crumbs. The butter pieces should be visible — this creates a tender, crunchy topping.

Assemble the crumble: Spread the apple mixture evenly in the prepared baking dish. Sprinkle the crumble topping over the apples, covering them completely.

Bake: Place the dish in the oven and bake for 40–45 minutes, or until the topping is golden brown and the apples are tender when pierced with a fork.

Cool slightly: Let the crumble cool for 10–15 minutes before serving. This helps the juices thicken slightly and makes it easier to scoop.