01 - Preheat oven to 350°F. Line a baking sheet with parchment paper.
02 - Melt 1 tablespoon butter in a small saucepan over medium heat. Add diced apples, 2 tablespoons brown sugar, 1 teaspoon cinnamon, nutmeg, and lemon juice. Cook for 3-4 minutes until apples are just tender. Stir in cornstarch and cook 1 more minute until slightly thickened. Remove from heat and let cool.
03 - In a medium bowl, whisk together gluten-free flour, baking powder, salt, and 1/2 teaspoon cinnamon.
04 - In a large bowl, cream softened butter, granulated sugar, and 1/4 cup brown sugar until light and fluffy. Beat in egg and vanilla extract.
05 - Gradually add dry ingredients to wet ingredients, mixing until just combined.
06 - Scoop tablespoon-sized portions of dough onto prepared baking sheet. Use thumb or back of spoon to make an indent in center of each cookie.
07 - Spoon a small amount of apple filling into each indentation.
08 - For topping, mix oats, brown sugar, and cinnamon. Sprinkle over apple filling.
09 - Bake cookies for 15-18 minutes until edges are lightly golden.
10 - Allow cookies to cool on baking sheet for 5 minutes, then transfer to wire rack to cool completely.