- Preheat the oven to 200°C.
- Roll the pastry out to approx. 0.25cm thick and cut 24 circles from it, using a 5cm pastry cutter.
- Place the circles onto a baking tray and prick all over with a fork.
- Bake in the oven for 10-12 minutes, or until cooked through and golden-brown. Remove the pastry from the oven and set aside to cool.
- Heat the oil in a frying pan and fry the onions over a low heat for 15 minutes, stirring regularly.
- Increase the heat, add the garlic, thyme and tomatoes and cook for a further 5 minutes.
- Add the vinegar and brown sugar and cook, stirring well, until caramelised. Season to taste with salt and freshly ground black pepper.
- To assemble the tartlets, spoon some of the tomato and onion mixture into each pastry circle and top with a piece of brie.