Preheat the oven to 350°F and prepare an 8-inch cake or springform pan by lining it with parchment and greasing it with butter or baking spray.
In a large bowl, whisk together the all-purpose flour, almond flour, baking powder, salt, and cinnamon.
In a separate bowl, vigorously whisk the eggs for about a minute until frothy.
Add the sugar, melted butter, and vanilla extract, and whisk again until smooth.
Pour the dry ingredients into the wet mixture and gently fold together with a rubber spatula until only a few flour streaks remain.
Fold in half of the apple slices, then pour the batter into the prepared pan, smoothing the top with an offset spatula.
Arrange the remaining apple slices on top, skin-side up, and sprinkle with sliced almonds.
Bake for 50–55 minutes, or until a tester inserted into the center comes out clean.
If the top browns too quickly, cover with foil during the last 20 minutes.
Allow the cake to cool in the pan for 20 minutes before inverting onto a plate, then flipping onto a platter.
Dust with powdered sugar.
Serve warm or let it cool completely before slicing.