For the Batter: Preheat oven to 425 °F. Whisk together oat flour, brown rice flour, baking powder, salt, and ground cinnamon in medium bowl. In small bowl, whisk together eggs, milk, melted butter, and vanilla. Pour egg mixture over dry ingredients. Whisk until smooth. Batter will be thin. Allow batter to stand for ten minutes.

For the Apples: Whisk together granulated sugar, cinnamon, and nutmeg in a small bowl. Set aside. Melt butter in a 10-inch oven-safe skillet (I used cast iron) over medium heat. Sprinkle sugar mixture over melted butter. Stir with a wooden spoon to combine. Cook until sugar begins to melt. Reduce heat to medium-low and carefully place apple slices in a circular pattern in the pan. Cook until sugar mixture begins to bubble up around apple slices, about three minutes. Turn off heat.

Slowly pour batter evenly into the pan. Some of the apple pieces might float and move. That’s fine. Transfer pan to the preheated oven. Bake until pancake puffs and is golden brown, about fifteen minutes.

Remove pan from oven. Slice pancake into wedges. Serve warm.