peel the skin off the onion and cut the ends off then cut the onion in half and thinly slice it into strips, heat the olive oil and butter in a wide pan and add the onions and sautee for a few minutes, stirring to evenly colour the onions. Add a pinch of salt.
when the onions are soft and have a lovely caramelised appearance add a good splash of balsamic vinegar and reduce for a few more minutes. Be careful that the pan does not spit when you add the balsamic vinegar. remove from the pan and add to a bowl to cool
Mix in the thyme, soft goats cheese and season with black pepper and some more salt,
Preheat the oven to 180c/350f
slice the tomatoes, you may not need all of the 1kg i suggested it depends what variety you use and how you slice them. I kept some of the small piccolo cherry tomatoes whole,
Roll out the block of puff pastry on a lightly floured surface into a large circle or rectangle, it needs to be around 4-5 mm thick. you can also use a pack of ready rolled puff pastry.
Place the rolled out pastry onto some baking paper. Spread the onion and goats cheese filling onto the middle of the tart leaving space round the edge to allow you to fold the edges over.
Arrange the sliced tomatoes on top, then fold the edges over to seal the tart.
Brush the folded over edge with a beaten egg and add salt and pepper and some more thyme or add some everything bagel seasoning onto the pastry.
Arrange some more thyme sprigs onto the tart tucked into the tomatoes, then add some flaked salt and cracked black pepper.
Bake the tart for about 40/45 minutes until the pastry is golden and puffed up.
Serve warm or chilled