Lemon Filling

Combine the egg whites, sugar, grated lemon zest, and lemon juice in a saucepan

Whisk gently and place the saucepan over simmering water

Cook for 5 minutes, stirring constantly, until thickened

Add 3 tablespoons of that warm mixture to the lightly beaten egg yolks and whisk vigorously

Immediately add the yolk mixture to the ingredients in the saucepan and cook for an additional 5 minutes, stirring constantly

Remove from heat and let cool completely before using.