In a medium saucepan bring water and agave syrup just to a boil, stirring until syrup is dissolved. Remove from the heat and cool for 10-15 minutes.

Wash the sorrel and shake dry. Roughly chop the sorrel, removing the ends of the stems and any leaves that are discolored or wilted. Add the sorrel to the syrup and let stand for 30 minutes.

Pour through a fine mesh sieve, discarding sorrel. Don't worry about the less-than-appetizing color.

Zest the whole lemon into the mixture. You can also use a peeler and sharp paring knife or the fine side of a box grater for this step. Slice in half with a serrated knife and squeeze the juice into the mixture through the sieve, catching any seeds. Stir in the tequila, then refrigerate until very cold.

Pour into an ice cream maker and follow the instructions to freeze it. If you do not have an ice cream maker, follow these directions to create an Italian-style ice or granita.