Preheat the oven to 180C/350F. Grease a 9-inch cake pan and set aside.
In a large mixing bowl, combine your almond flour, egg yolks, sugar, and lemon zest, and mix well. In a separate bowl, beat together your egg whites until stiff peaks form.
Fold through your egg whites until combined. Transfer the cake mixture into the greased cake pan and top with sliced almonds.
Bake the cake for 30 minutes, or until tender on top. Remove the cake from the oven and let cool for 15 minutes, before carefully invert onto a plate and sprinkling with powdered sugar.
* You can use a brown sugar substitute for a keto option.TO STORE: Leftover cake can be stored at room temperature, covered, for up to 1 week. You can store the cake in the refrigerator if you'd like it to keep longer. TO FREEZE: Place the cake in a shallow container and store it in the freezer for up to 2 months.