Preheat the oven to 220°C, fan 200°C, gas 7.
On a floured surface, roll out the pastry to a thickness of 4mm, then cut out 4 circles slightly larger than 10cm tartlet tins. Chill in the fridge for 10 minutes.
Meanwhile, heat the balsamic vinegar and sugar in a small saucepan, stirring until the sugar dissolves. Simmer for about 2 minutes, until slightly syrupy. Remove from the heat and add the butter and thyme, stirring until the butter has melted.
Divide the sauce between 4 tartlet tins. Place 3 vine tomatoes (tops facing down) into the balsamic caramel in each tin. Top with the pastry circles and gently tuck in the sides.
Place the tins on a baking tray and cook for 20 minutes. Allow to rest for 2-3 minutes before turning out each tart onto a plate to serve. Great served hot or cold and delicious with a salad.
Cook's tip: when rolling pastry, make sure you turn it constantly as you go – rolling pasty in the same direction causes it to shrink.