In heavy-bottomed saucepan, whisk together eggs, yolks, sugar, and lemon juice. Stir in butter. Cook over medium-low heat, stirring constantly with a heat-proof rubber spatula, until mixture is thick enough to coat back of a spoon, about 10 minutes. (Do not allow mixture to come to a boil or it will curdle). Whisk in half-and-half; continue to stir over medium-low heat until thick, 3 to 5 minutes. Strain custard into bowl. Stir in zest.