Put the sugar into a small bowl, pour over 1tbsp of boiling water and stir to dissolve. Don't worry if it's still a bit grainy. Halve the vanilla pod lengthways and scrape the seeds into the syrup. Add the pod, too, and the lemon zest. Leave to infuse for at least 15 minutes.
Reserve 2-3 small strawberries to decorate, hull, halve, then coarsely mash the rest using either a stick blender or food processor, leaving some little chunks of fruit still intact. Remove the vanilla pod and lemon zest from the syrup, then stir it into the strawberries to make a rough purée.
Whip the cream into stiffish peaks then stir in the fromage frais. Fold all but a couple of spoonfuls of the strawberry purée through the cream mixture. Swirl in the saved purée to give a rippled effect. Spoon into 4 large or 6 smaller glasses. Halve the reserved strawberries. Top each glass with a spoonful of clotted cream and a halved strawberry.
You can prepare this up to a day in advance if you like - just add the halved strawberries at the last minute.