Bring the cream, half-and-half, sugar and zest just to a boil over medium heat in a heavy medium saucepan, stirring to dissolve the sugar.
Whisk the egg yolks and salt together in a medium bowl. Add 1/2 cup of the cream mixture to the yolk mixture and whisk until blended.
In a slow, steady stream, add the remaining cream mixture, whisking constantly, and continue whisking until blended. Return the mixture to the saucepan and cook, stirring, over medium-low heat until the custard thickens and leaves a path on the back of a wooden spoon when a finger is drawn across it; do not allow the mixture to boil.
Immediately pour the custard through a strainer into a bowl and stir in the vanilla. Let cool to room temperature, whisking occasionally.
When ready to freeze, stir the lemon juice into the cold custard, pour the mixture into an ice cream maker and freeze according to the manufacturer's instructions. The ice cream will be soft but ready to eat.