Whisk the sugar, lemon juice, lemon zest, egg yolks, and whole eggs together in a metal bowl. Place the bowl over a pot of simmering water and gently whisk the mixture by hand (you do not need to whisk quickly, and can even step away now and again), until thickened to the point that the curd leaves a ribbon on top of itself when the whisk is lifted, about 20 minutes.