Heat the oven to 325 degrees
Butter an 8 inch round cake pan. Line the bottomwith parchment round Butter the parchment. Lightly dust with almond flour.
Beat the egg whites in a clean bowl with the stand mixer or a hand-held electric mixer until soft peaks form. Gradually add the remaining half of the sugar to the egg whites mixture and beat until stiff peaks form. Put meringue in a clean bowl and keep it in the refrigerator until needed.
Zest the lemons and add the zest to half the sugar. Using your fingers, gently press the lemons into the sugar until fragrant.
Place the butter, the sugar with the lemon zest, and vanilla in an electric stand mixer and beat for 8-10 minutes or until pale and creamy. Scrape down the sides to fulling incorporate the sugar into the butter
Scrape down the sides of the bowl, then gradually add the egg yolks, one at a time, continuing to beat until fully combined. Scrape down the mixture between adding egg yolks.
Using a hand mixer, beat the ricotta until slightly fluffy. This will help make the cake even lighter.
Add the almond meal to the butter mixture and beat to combine. Fold ricotta into the almond meal mixture.
Gently fold ⅓ of the egg whites into the cake mixture. Repeat with the rest of the egg whites. You want to incorporate all of the egg whites without leaving white streaks or blobs in the cake mix but you don't want to over-fold and flatten the meringue.
Pour the mixture into the prepared cake tin, smooth the top with a palette knife or offset spatulata, but do not deflate. Decorate the cake with slivered almonds and bake for about one hour or until cooked and firm to touch. Allow to cool completely in the cake tin.
TO SERVE:Dust with powdered sugar or add fresh blueberries (or both!). Serve cake at room temperature. If you want a more cheesecake-like texture, chill in the refrigerator. Can can be stored in an airtight tin in the refrigerator for up to five days.