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Heat oven to 200C/180C fan/gas 6. Unroll the pastry sheet onto a large baking tray (leave it on the paper from the pack). If the edges are not straight, trim them with a sharp knife. Score a border, about 1cm in from the edge. Score a criss-cross pattern over the central piece of pastry (this will prevent it from rising too much) and around the border, if you like. Bake for 15 mins.
Meanwhile, heat the oil in a large pan. Add the onions and cook for 10-15 mins until softened and starting to caramelise. Stir in the vinegar, bubble for 1 min more, then add the Dijon mustard and thyme, season well and set aside. Slice the apple thinly through the core so a few slices have a nice star in the middle, and remove any seeds. Toss the slices in a splash of vinegar to prevent them from browning.
The pastry should be puffed and starting to colour. Push down the centre, then spread over the onions. Add blobs of cranberry sauce, then top with the apple slices and brie, overlapping them to cover the tart. Add a few more thyme sprigs, drizzle with a little oil, then put back in the oven for a further 25 mins until the cheese is bubbling and the apple slices are soft. Serve warm or cold.
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