Prepare the apples: core, cut in half, and slice them into thin slices.
Add the slices into a large bowl along with the lemon juice. Carefully toss to combine. Set aside.
Preheat the oven to 350°F/ 180°C.
Line an 8-inch springform or round pan with parchment paper and grease with butter. Set aside.
In a medium bowl, combine the flour, salt, and baking powder. Set aside.
In a large mixing bowl, add the butter and sugar. Beat on medium high speed for 2-3 minutes, or until light and fluffy.
Add in the eggs, one at a time, mixing thoroughly before adding the next. Add in the vanilla, scraping the sides and bottom as needed.
Beat in the almond flour. Add in the flour mixture, beating just until combined.
Transfer half the batter to prepared pan, using a flat spatula or spoon to smooth out the top. Arrange half the apple slices in a layer, slightly overlapping. Sprinkle with half the sliced almonds.
Spread the remaining batter over the apple layer, then top with remaining apple slices. Sprinkle with remaining sliced almonds.
Bake for 40-50 minutes, or until a toothpick inserted in the center comes out clean.
Serve dusted with powdered sugar and vanilla custard, if desired. For a quick glazing, melt about 2 tbsp fruit jam with 1/2 tbsp water in a small saucepan. Brush the glaze over the cake for a shiny finish.