Prepare the cake pan. Preheat an oven to 350F. Line an 8-inch cake pan or springform pan with parchment, then spray generously with baking spray or grease with butter.
Mix the dry ingredients. In a large bowl, whisk together ¾ cup (90g) all-purpose flour, ⅓ cup (35g) almond flour, ¾ tsp baking powder, ½ tsp kosher salt, and ¼ tsp ground cinnamon.
Mix the wet ingredients. In a separate large mixing bowl, vigorously whisk 2 large (100g) eggs until frothy, about 1 minute. Add ¾ cup (150g) sugar, 8 Tbsp (113g) melted butter, and 2 tsp vanilla extract, then whisk until thoroughly mixed.
Finish the cake batter. All at once, pour in the dry ingredients, then use a rubber spatula to gently fold them into the wet ingredients. When there are just a few streaks of flour remaining, add half of the apple slices. Fold again gently until just mixed, then pour out into the prepared cake pan. Use an offset spatula to smooth the cake into an even layer, then top with the remaining apple slices. You can arrange them in any way you like: concentric circle, rows of apples, whatever you like. I recommend facing them skin side up. Sprinkle with ⅓ cup (30g) sliced almonds.
Bake the cake. Transfer the cake to the preheated oven then bake for 50-55 minutes, or until a cake tester inserted into the center of the cake comes out clean. *If the cake begins to brown too quickly, you can cover it with aluminum foil for the last 20 minutes of bake time.
Cool, then serve! If serving warm, allow the cake to cool in the pan for 20 minutes, then transfer to a platter by inverting first onto a plate, then back onto the platter. Dust with powdered sugar, then slice and serve. Alternatively, you can allow the cake to cool completely before slicing. Enjoy!