Bring the water, orange juice and lemon juice to the boil with the sugar, boil fast for approx. 1 min., leave to cool. Add the herbs to the syrup, puree. Pour the mixture into a stainless steel bowl, cover and freeze for approx. 12 hrs., stirring thoroughly 4 times. Puree approx. 1 hr. prior to serving, freeze again.
Unroll the puff pastry, cut out 6 circles (each approx. 12 cm in diameter), transfer to a baking tray lined with baking paper and prick all over with a fork. Place the rhubarb on top of the pastry dough, brush with oil, sprinkle with sugar.
Approx. 25 mins. in the lower half of an oven preheated to 200°C. Remove from the oven, allow the tarts to cool a little on a rack and serve with the herb sorbet.