Preheat the oven to 200C/400F/Gas 6. Make a shallow cut in the puff pastry 1cm/½in in from the edge and all the way around.
In a pan cook the onions, tomatoes, garlic and thyme in the butter and olive oil for 20 to 30 minutes till caramelised.
Add the balsamic and sugar and cook for a couple of minutes, then spoon the onion into the centre of the pastry, leaving the scribed edge clear.
Dot the top with goats' cheese, brush the edge with beaten egg and bake for 20 minutes.
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