Peel the nectarines, reserving the peelings, and slice them, reserving the pits. In a food processor combine the nectarine slices, 1/2 cup of the sugar, and the lemon juice and pulse the motor until the nectarine slices are chopped coarse. Transfer the mixture to a bowl and chill it, covered. In a saucepan heat the half-and-half, the cream, the reserved nectarine peelings and pits, the remaining 1/4 cup sugar, the vanilla bean, the zest, and a pinch of salt over moderate heat until the mixture is very hot but not boiling;. Remove the pan from the heat, let the mixture cool to room temperature, and chill it, covered, for 1 hour, or until it is very cold.
Strain the cram mixture through a fine sieve into the bowl with the chopped nectarines, stir the mixture until it is combined well, and freeze it in an ice-cream freezer according to the manufacturer's instructions.