Put the lemon zest and sugar in a food processor and process until the zest is finally chopped. In a saucepan, mix the lemon sugar with 1 1/2 cups heavy cream and all milk. Bring to a boil, stirring occasionally to dissolve the sugar. Place the egg yolks in a large bowl and whisk briefly. still whisking the yolks, slowly pour in the hot cream. When the mixture is smooth, pour it back into the saucepan or into the top of a double boiler. Cook over low heat or over simmering water, stirring constantly, until the mixture becomes a thick custard, about 15 min. Do not let the mixture boil. Place the custard in a metal bowl set over a larger bowl of ice. Stir until very cold and thick. Mix in the lemon juice. Whip the remaining cup of cream until stiff. Fold in the lemon custard. Pour the mixture into the bowl of the machine and freeze.