Combine the water, sugar, and lemon verbena in a medium, heavy saucepan, and bring to a boil. Stir the pot until the sugar dissolves, and then reduce the heat to medium-low. Allow the pot to simmer for 5 minutes. Remove from the heat and allow the herb-syrup to steep while it cools. Refrigerate for 2 hours, or until thoroughly chilled. Strain, add the lemon juice, and process in an ice cream machine according to the manufacturer's instructions. Transfer the sorbet to an airtight container and let firm in the freezer for at least 2 hours before serving.