Preheat your oven to 350°F (175°C).
Peel, core, and slice the apples into thin wedges. Place them in a large bowl and toss with the sugar, brown sugar, cinnamon, nutmeg, vanilla, lemon juice, and flour.
Spread the apple mixture evenly in a greased 9×9-inch baking dish or similar oven-safe pan.
In a separate bowl, combine the flour, oats, brown sugar, cinnamon, and salt.
Pour in the melted butter and stir until the mixture forms coarse crumbs.
Sprinkle the crumble evenly over the apples, covering the entire surface.
Bake for 40–45 minutes, until the top is golden brown and the apple filling is bubbling around the edges.
If the topping starts browning too quickly, cover loosely with foil during the last 10 minutes of baking.
Let the crumble cool for at least 15 minutes before serving.