Dissolve 25 g sugar in 125 ml
water in a saucepan. Add the contents of one sachet. Stir until
completely dissolved. Bring the mixture to the boil, then maintain at
a gentle boil for 1 min. Spread the mixture evenly over the cooled
tart using a soft brush. The topping will set in
1 h at room temperature and in 15 min in the fridge.