For the Filling:
3 pounds apples peeled, cored, and cut into 1/4 inch slices
1 tablespoon lemon juice
1/3 cup granulated sugar
1/4 cup all-purpose flour
1 teaspoon ground cinnamon
1/4 teaspoon ground nutmeg
Pinch of salt
For the Topping:
1 cup rolled oats
1 cup light brown sugar (lightly packed)
1 cup all-purpose flour
1 cup cold butter, cut into small dice
Preheat the oven to 375°F with a rack in the center. To make the filling, combine the apple slices with the lemon juice in a large mixing bowl and toss to coat. Add the sugar, flour, cinnamon, nutmeg, and salt, and toss gently until apples are evenly coated with other ingredients. Set a deep-dish pie plate or 2-quart casserole on a baking sheet and pour filling into pie plate or casserole.
To make the topping, combine the oats, brown sugar, flour, and butter in a small bowl. Pinch all ingredients together with your fingers until it forms a coarse crumble with no loose flour, oats, or sugar. Distribute topping evenly over apple mixture. Bake until topping is browned and filling is hot and bubbly, about 65 minutes. If topping starts to brown too much, cover it loosely with aluminum foil for the duration of baking. Let cool for 10 minutes or so and serve warm.