Preheat your oven to 350°F (175°C).

In a large bowl, toss the apple slices with granulated sugar, cinnamon, nutmeg, and lemon juice until well coated.

Spread the apple mixture evenly in a greased 9×13-inch baking dish.

In another bowl, combine the flour, brown sugar, melted butter, oats, and salt. Mix until the mixture resembles coarse crumbs.

Sprinkle the crumb mixture evenly over the apples in the baking dish.

Bake in the preheated oven for 40-45 minutes, or until the top is golden brown and the apples are tender.

Allow the casserole to cool slightly before serving. Enjoy warm.

For a nutty twist, add 1/2 cup of chopped walnuts or pecans to the crumb topping. If you prefer a less sweet version, reduce the amount of granulated sugar to 3/4 cup. You can also experiment with different types of apples for varying levels of sweetness and tartness. For a gluten-free option, substitute the all-purpose flour with almond flour or a gluten-free baking mix.