INGREDIENTS

For the tartlets:
1 large sheet good quality puff pastry
1 free-range egg yolk, mixed with a splash of milk
4 heaped tablespoons caramelised onion jam (see below)
175g good-quality goat’s cheese, cut into 1.5cm thick rounds
3-4 thyme spring, leaves picked, plus extra sprigs to garnish
Sea salt and freshly ground black pepper
250ml balsamic vinegar (use the best you can afford)
3 tablespoons brown sugar

To make the tartlets: 

Preheat the oven to 200˚C (fan-forced), 220˚C
Using a 12cm round pastry cutter or a small bowl as a template, cut four rounds of pastry and place on a large non-stick or lined baking sheet. Score a 2cm border around the edges of the rounds, being careful not to cut all the way through. Avoiding the border, prick the bases all over with a fork. Brush the borders with the egg wash, taking care not to let the egg run down the sides or the pastry will rise unevenly.
Again, avoiding the border, divide the caramelised onion jam among the bases, spreading it out evenly. Place a round of goat’s cheese on top, then scatter with the thyme leaves and season with pepper.
Bake for 20 minutes or until the puff pastry borders are golden and risen and the bases of the tartlets are cooked.
Mean while, place the balsamic vinegar in a small saucepan and bring to the boil, then reduce the heat and simmer until reduced by half. Add the brown sugar and simmer until the mixture becomes syrupy – it should coat the back of a spoon when it’s ready. Set aside to cook and thicken.
Serve the tartlets drizzled with the balsamic syrup and scattered with thyme sprigs.

http://www.whatkatieate.com/recipes/caramelised-onion-and-goats-cheese-tartlets-with-balsamic-syrup/