To clean the pearl barley, place it in a fine mesh strainer and pour boiling water over it. Drain.

Combine the drained barley, water, and lemon peel in a medium saucepan, and bring to a boil. Reduce the heat to low and simmer for 25 minutes. Remove from the heat and strain into a clean container, adding fresh water to bring to 4 cups in volume. Add the lemon juice and honey to taste, and stir well to combine. Refrigerate until well chilled.

Serve chilled, mixed with gin, if using, and garnished with lemon slices.