In a medium, heavy saucepan, bring the cream, half-and-half, sugar, and zest to a boil, stirring occasionally until the sugar is melted. Remove from the heat.
In a medium bowl, beat the egg yolks until thick and pale, about 2 minutes. In a slow stream, whisking constantly, add about 1 cup of the hot cream mixture to the egg yolks. Beat until well incorporated. Return the yolk mixture to the saucepan with the hot cream and bring to a bare simmer. Cook until a thick custard forms, and the mixture reaches 170 degrees on an instant-read thermometer, about 6 minutes. Remove from the heat and strain through a fine-mesh strainer into a clean container. Stir in the lemon juice and limoncello, and cover with plastic wrap, pressing down against the custard to prevent a skin from forming. Refrigerate until well-chilled, about 2 hours.
Transfer to an ice-cream maker and process according to the manufacturer's instructions. Transfer to an airtight plastic container and freeze until ready to serve.
Yield: 1 quart