Preheat the oven to 220'C/425F/Gas Mark 7.
Lightly grease a baking sheet and lay the puff pastry sheet on top. Heat the butter and oil in a frying pan and add the red onions. Cook, stirring occasionally for 5 minutes until the onions are very soft and caramalised. Season with salt and pepper and add the dried herbs. Cool for a few minutes.
Spread the onions over the pastry sheet to within 2cm (3/4 inch) of the edges. Scatter the cheese on top, then add the rosemary and thyme sprigs. Transfer to the oven and bake for 12 - 15 minutes until puffed up and golden brown. Cool for a few moments, then slice and serve.
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