Preheat your oven to 175 °C (350 °F).
In a large bowl, mix together the flour, oats, cinnamon, nutmeg, and brown sugar. Then, using a knife or pastry cutter, work in the butter until the mixture is crumbly.
Spoon half of this crumbly mixture into the base of a 9x13-inch baking dish and flatten it down with a spoon to create an even layer.
Arrange the apple slices on top of the flattened crumbs, followed by sprinkling them with the remaining crumb mixture.
Bake in the preheated oven until the apples are tender, which should take around 45 to 50 minutes.
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