Preheat oven to 400°F. Line bottom of a 9″ springform pan with baking paper. Lightly coat paper and sides of pan with cooking spray.
Separate eggs. Reserve 3 yolks, and discard 4 yolks. Set aside.
In small bowl, beat egg whites and salt with electric mixer until stiff peaks form. Set aside.
In separate mixing bowl, beat 1/2 cup of the sweetener and 3 egg yolks until thick. Add lemon juice.
With mixer on low-medium setting, beat in cream cheese, a little at a time, until well blended and fluffy.
Add grated lemon peel, and mix in gently.
Gently fold egg whites into cream cheese mixture a little at a time, pulling egg whites up through cream cheese mixture.
Pour mixture into pan. Shake gently to level batter.
Bake 10 minutes at 400°F. Reduce temperature to 300°F and bake for another 40 minutes. When top of cake is set, turn off oven and keep oven door closed. Allow to cool for another hour in oven. Remove and continue to cool on a wire rack.
To make the topping: In small bowl, whisk 2 tablespoons of sweetener with sour cream and vanilla extract. Spread on top of cheesecake when completely cooled.