Grease/butter a 9" round cake pan or 9" springform pan. Line the bottom with a circle of parchment paper. Preheat oven to 350˚F.

In a large mixing bowl, combine 4 egg yolks and 1/2 cup sugar. Beat with an electric hand mixer on high speed 3 minutes or until thick, creamy, and lightened in color. Add 1 1/2 cups almond flour and 1 Tbsp lemon zest, and use a spatula to fold and stir together until incorporated. Wash and dry attachments from the mixer.

In a second large mixing bowl, add 4 egg whites and beat together until stiff peaks form (about 2 minutes). Fold the egg whites 1 heaping spoonful at a time into the almond batter, folding with each addition just until incorporated and scraping from the bottom of the bowl to ensure the batter has an even consistency.

Once all the egg whites are incorporated and the batter is even in consistency (do not over-mix), transfer the batter to the prepared baking pan and sprinkle the top with 1/4 cup sliced almonds. Bake at 350˚F for 30 minutes or until top is firm.

Let the cake rest in the pan for 15 minutes. Then run a thin spatula around the sides and invert it onto your hand, and then place it right side up on the rack. Let cool to room temperature and garnish with powdered sugar and lemon slices if desired.