In a large mixing bowl, combine gluten-free all-purpose baking flour, salt, and granulated sugar. Cut in cold, cubed unsalted butter until the mixture resembles coarse crumbs. Gradually add ice water, mixing until a dough forms. Shape the dough into a ball, wrap it in plastic, and chill in the refrigerator for at least 30 minutes.

Prepare the apple filling by combining peeled and finely diced apples with light brown sugar, ground cinnamon, nutmeg, and gluten-free all-purpose flour. Stir gently to coat the apple pieces evenly, then set aside.

Preheat your oven to 425°F (220°C).

Roll the chilled dough out to a thickness of 1/8–1/4 inch. Cut 4-inch circles from the dough and transfer them to a greased muffin tin.

Fill each dough-lined muffin cup with the prepared apple filling, packing it in gently. Brush the edges of the crusts with egg wash or plant milk.

Bake for 20-24 minutes until the crusts are golden brown and the filling starts to bubble. Let cool before transferring to a wire rack.