It's actually called 'mirror glaze' or 'miroir glaze' (I think that's French). It's quite hard to find and when you do it's usually sold in bulk (e.g. 6kg) as it's only really used by professional pastry chefs.
I am currently making lemon curd tarts and want to get that beautiful glossy finish so I plan on experimenting with something like this (note that it's an experiment!):
- gold gelatine leaves
- lemon juice
Bloom the gelatine leaves in some cold water.
In a saucepan, add quantities of lemon juice, sugar and water to your taste and bring to the boil or until sugar dissolves.
Once the above has cooled slightly, add the bloomed gelatine leaves and stir until dissolved. Set aside until mixture begins to thicken, then pour/spoon over filled tarts.
Note that with with gelatine leaves, the proportion is 2g gelatine leaves to every 100mL liquid.