For the lemon sorbet, bring the water to the boil, add the sugar and stir to dissolve.
Allow to simmer for at least 5 minutes until the sugar water takes on a syrupy consistency. Then remove from heat and let sit for 30 minutes.
Meanwhile, squeeze the lemons and grate the zest from one of them. Blend in lemon juice and cinnamon using a hand blender and refrigerate overnight.
Blend mixture in ice cream maker according to instructions. Put the lemon sorbet in the freezer for 2 hours and serve garnished with lemon balm.