We love onions and tarts. Lets combine them and rejoice!
Heat your oven to 400 F. Gently heat the oil and butter in a large pan, then add the halved onions, cut side down. Cover with a lid and cook until the onions are well softened.
Remove the lid and season well with salt and pepper. Turn the onion over, then cook for a further 5 minutes until they begin to turn golden. Carefully turn them back over, add the chopped thyme and vinegar, then cook for another 5 minutes. Remove them from heat and allow them to cool slightly.
Cut your pastry dough into 4 big squares. Place an onion half, cut side up, on each square. Add a cheese wedge if you chose to do so and using a little dab here and there of water, fold the pastry up around the onion and cheese, making a little parcel-type thing. Bake for about 20 minutes or until the dough is fully cooked. Serve with your favorite veggies on the side!