Rinse raspberries (set some raspberries aside for garnish), puree in a blender and press puree through a sieve.
Boil water and sugar in a saucepan and then let cool slightly. Add raspberry purée to the saucepan, mix and then pour into a shallow metal bowl. Let rest in the freezer for about 2-3 hours. Stir vigorously every 30 minutes.
Or freeze in an ice cream machine to a smooth granita. Serve scooped in a small bowl garnished with raspberries and lemon zest.