Prepare the pan. Preheat an oven to 375° (190°C). Line a 9-inch round pan with parchment paper, then coat the bottom and sides with baking spray (or butter).
Whip egg whites. Place 6 large egg whites in the bowl of a stand mixer fitted with whisk attachment. Beat at medium-high speed until stiff peaks form, about 3-5 minutes. Transfer to a separate bowl, then set aside.
Cream eggs and sugar. Swap the whisk attachment for the paddle attachment, then combine 6 large egg yolks and ¾ cup (150g) sugar in the stand mixer bowl. Beat on medium to medium-high speed until light and fluffy, about 2-3 minutes.
Add dry ingredients. Sift in 2 cups (200g) almond flour and ½ tsp kosher salt, then beat until well mixed. Use a spatula to scrape down the sides as needed.
Add lemon. Add 2 tsp lemon zest, 4 Tbsp (60ml) fresh lemon juice, and 1 tsp almond extract, then beat until well combined.
Fold in egg whites. Use a spatula to gently fold about ⅓ of the whipped egg whites into the batter. Repeat with remaining egg whites until fully incorporated and no streaks remain. Carefully transfer the batter to the prepared cake pan, then use the spatula to smooth out on the top.
Bake. Sprinkle ½ cup (45g) sliced almonds right on top. Transfer to the oven and bake for 20-25 minutes, or until a cake tester inserted into the center of the cake comes out clean.
Serve. Allow the cake to cool completely (you can of course enjoy it warm, but it’s easier to slice once it cools). If using a springform pan, release the mold to remove the cake. Otherwise, you can run a paring knife along the edges of the cake before inverting onto a serving tray. Before serving, dust with confectioner’s sugar. Slice into 8 pieces, then enjoy!