Preheat the oven to 200ºC, gas mark 6.
Place the baby beetroots in a roasting dish.
Mix the vinegar, sugar, oil, thyme, salt and pepper and pour over the beetroots.
Cover the tray with foil and roast for 50 minutes.
Remove the foil and cook for a further 10 minutes.
Peel the beetroots while they are still warm and slice into 5mm rounds. Set aside.
Heat the balsamic vinegar and butter in a 23cm non-stick frying pan over a medium heat. Bring to a boil and cook for two minutes.
Remove from the heat and place the beetroot slices across the base of the frying pan.
Using the pan as a guide, cut a 23cm circle from the puff pastry and place on top of the beetroot, tucking it in around the edges.
Bake for 30-35 minutes or until puffed and the pastry is deep golden.
Carefully invert onto a large plate and serve.