Preheat oven to 350 degrees Fahrenheit.
Core the apples and slice in half. Place in an oven safe baking dish, cut side up, and set aside.
In a medium sized bowl, combine brown sugar, gluten-free flour, gluten-free certified oats, ground cinnamon, ground nutmeg, and salt. Whisk together.
Using your hands, work the butter into the mixture. You'll want some pieces of butter to remain in tact, about the size of peas.
With a spoon, fill the apples with the brown sugar mixture.
Add 1/2 cup of water to the bottom of the baking dish.
Bake in the oven at 350F for 45-50 minutes, or until the apples are soft and the brown sugar mixture is golden.
Let cool for a few minutes before serving. Serve with ice cream if desired.