1. Slice you onions into large ribbons. Put them into a frying pan with the oil on a low/ medium heat.
2. Fry the onions gently and keep stirring them until they are very soft (if you try one there should be no crunch at all).
3. While the onions fry roast you nuts in a small pan, lay the pastry and the paper on a baking tray and slice your brie.
4. Score the pastry around the outside about half a thumb from the edge and score with a knife.
5. Preheat the oven to 200C.
6. When the onions are soft (15-25 mins) pour in the balsamic vinegar and cook until all the liquid has evaporated
7. Then add the sugar and caramelise the onions (they should be dark and sticky now).
8. Pour the onions onto the large inside rectangle and scatter the slices of bries around the tart.
9. Paint the outside with egg and scatter sea salt all over the tart - particularly on the outside for a delicious crust
10. Bake for 18-20 mins or until golden brown.
11. Scatter your roasted nuts over the tart along with the thyme and cracked black pepper.