Start by preheating your oven to 350°F (175°C). This ensures that your cake bakes evenly and rises beautifully.
Grease your 9-inch cake pan with a little butter or oil, and line the bottom with parchment paper for easy release.
Carefully separate the egg yolks from the egg whites. Place the yolks in one mixing bowl and the egg whites in another. Make sure there is no yolk in the whites, as this will prevent them from whipping properly.
Using an electric mixer, beat the egg whites on medium speed until they become frothy. Gradually add in half of the granulated sugar, continuing to beat until stiff peaks form. This should take about 3-5 minutes.
In the bowl with the egg yolks, add the remaining granulated sugar and lemon zest. Whisk together until the mixture is pale and fluffy.
Gently fold the almond flour into the egg yolk mixture until just combined. Then, carefully fold in the whipped egg whites in three additions. Be gentle to maintain the airiness of the batter.
Pour the batter into the prepared cake pan, smoothing the top with a spatula. Sprinkle the sliced almonds evenly over the top. Bake in the preheated oven for 25-30 minutes, or until the cake is golden brown and a toothpick inserted in the center comes out clean.
Once baked, remove the cake from the oven and let it cool in the pan for about 10 minutes. Then, transfer it to a wire rack to cool completely. Before serving, dust the cooled cake with confectioners' sugar and garnish with lemon slices if desired.
