Bring water and honey to a boil in a pot over medium-high heat. Gently crush the lavender in a mortar and pestle or with the back of a knife. Add the crushed lavender to the pot, give it a good stir and then remove it from the heat.
Cover and refrigerate for at least 2 hours. Strain the liquid out through a sieve into a large bowl or pitcher and add the lemon juice. Give it a good stir then serve over ice with a sprig of rosemary.