01 - Preheat oven to 350°F. Grease a 9x13-inch baking dish thoroughly with butter or nonstick spray.
02 - In a large bowl, combine the sliced apples, granulated sugar, brown sugar, ground cinnamon, nutmeg, flour, lemon juice, and a pinch of salt. Toss until apples are evenly coated.
03 - Evenly spread the prepared apple mixture into the greased baking dish.
04 - In a separate bowl, whisk together flour, granulated sugar, light brown sugar, baking soda, cream of tartar, salt, and ground cinnamon.
05 - Add melted butter, egg, and vanilla extract to the dry ingredients. Stir until a soft, slightly sticky dough forms.
06 - Drop spoonfuls of the snickerdoodle dough over the apples. Gently spread to cover most of the filling, leaving some spaces where apples peek through.
07 - In a small bowl, mix together granulated sugar and ground cinnamon. Sprinkle evenly over the snickerdoodle dough.
08 - Bake in the center of the oven for 40 minutes, until the topping is golden brown and apples are bubbling beneath.
09 - Allow to cool for 10 to 15 minutes before serving. Enjoy warm, optionally topped with vanilla ice cream or whipped cream.