Preheat oven to 375°F (190°C). Grease an 8x8 inch baking dish.

Peel, core, and slice the apples. In a large bowl, toss apples with granulated sugar, kosher salt, cinnamon, nutmeg, lemon juice, and cornstarch until evenly coated.

Pour the apple mixture into the prepared baking dish.

In a medium bowl, combine the gluten-free oats, gluten-free flour, and brown sugar. Use your fingers or a fork to mix until crumbly and well combined.

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

Bake for 35-45 minutes, or until the topping is golden brown and the apples are tender and bubbling. Let cool for 10-15 minutes before serving.