Sift in the flour, baking powder, and ground cinnamon. Whisk until no dry ingredients remain and the mixture is smooth and lump-free. The batter should be thin.

Carefully fold in the apple slices, trying to coat each piece with the batter. Transfer the apples into the prepared pan in small batches, spreading each batch into an even layer. Make sure all apple slices are lying flat. You can arrange the top layer in a decorative pattern if desired.

Scrape the remaining batter and pour it over the apples. Bake for 1 hour on the middle rack. Rotate the pan, cover the top with foil if it's browning too much, and bake for an additional 30 minutes or until the toothpick inserted in the center of the cake comes out clean and dry.

Remove from the oven and let the cake cool in the pan until warm to the touch. Lift the cake out of the pan by holding onto the parchment paper and transfer it to a cooling rack to cool completely.

Cover the cake with plastic wrap and refrigerate for 1–2 hours before slicing and serving. Serve with whipped cream and a pinch of ground cinnamon or nutmeg, vanilla ice cream, or a drizzle of salted caramel.