Step 1

Combine the water, sugar, tarragon and grapefruit zest in a small saucepan over moderate heat. Stir until the sugar has dissolved. Remove from the heat and set aside to steep for 5 minutes.

Step 2

Add the grapefruit juice, tangerine juice and lemon juice to the sugar mixture. Strain with a fine sieve into a bowl, discarding the pulp and herbs. Let the mixture cool, then transfer into a container. Cover and freeze overnight. Let soften at room termperature before scooping.