Preheat the oven to 200°C/fan 180°C/gas mark 6. Bring a large pan of water to the boil. Add the sprouts and cook for 3 minutes. Drain and slice each one in half.
Roll the pastry out on a lightly floured surface and cut out a 25cm circle that's about the thickness of a £1 coin. Set aside in the fridge to chill.
Melt the butter in a 23cm ovenproof frying pan over a low-medium heat. Cook for 1 minute or until foaming. Raise the heat, add the sugar and vinegar and cook for 2 minutes.
Place the sprouts into the pan of butter and caramel so the round sides are facing down, then nestle in the sprigs of thyme and season to taste. Cover the sprouts directly with the pastry circle, tucking the edges tightly around. Bake in the oven for 30-35 minutes or until golden and puffed up. Turn on to a serving plate and finish with crumbled stilton.