Prepare the apples. Preheat your oven to 350°F. Thinly slice the apples, leaving the skin on or peeling them depending on your preference.
Make the caramelized topping. Add the butter, brown sugar, cinnamon, and salt to a skillet over medium heat. Stir until the butter melts and the sugar begins to dissolve. Add the sliced apples and cook for about 3–5 minutes, until they begin to soften and become glossy.
Arrange the apples. Grease an 8-inch round cake pan and carefully arrange the caramelized apples in an even layer across the bottom. Pour any remaining caramel mixture from the skillet over the apples.
Mix the dry ingredients. In a medium bowl, whisk together the all-purpose flour, almond flour, cinnamon, baking powder, and salt.
Mix the wet ingredients. In a separate bowl, whisk together the applesauce, milk, eggs, melted butter, and vanilla until smooth.
Combine. Slowly add the dry ingredients to the wet ingredients. Stir just until everything is combined. Avoid overmixing—the batter should be thick but pourable.
Add the batter. Pour the batter evenly over the caramelized apples. Gently spread the top with a spatula so the batter reaches the edges of the pan.
Bake. Bake for 30–35 minutes, or until the top is golden and a toothpick inserted into the center comes out clean.
Cool slightly. Allow the cake to cool in the pan for about 10–15 minutes. Don't let it cool completely, or the caramelized apples may stick to the pan.
Flip the cake. Place a serving plate over the cake pan. Carefully flip the pan upside down in one quick motion. Gently lift the pan away to reveal the caramelized apples on top.
Serve. Serve warm on its own or with a spoonful of Greek yogurt or vanilla ice cream.