Combine lemon zest, lemon juice and sugar into a mixing bowl. Gradually stir in milk, heavy cream and salt. Pour into an 8-inch square baking pan. Cover and freeze until solid around outside and mushy in the middle. Stir well with a wooden spoon. Cover and continue to freeze until firm. Serve in scooped-out lemon halves with a mint sprig.

(The ice cream, packed in a small metal bowl and covered airtight with plastic wrap and aluminum foil, will keep in the freezer for up to 1 week. For lime ice cream, substitute lime zest and lime juice. Can also be made in an ice cream maker following manufacturer’s instructions.)