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Heat the oven to 200C/180C fan/gas 6. Unravel the sheet of puff pastry on its baking parchment and slide onto a large baking tray. Score a border about 1cm from the edge all the way around the pastry, being careful not to cut all the way through. Prick the middle all over with a fork, then bake for 15 mins until puffed up and golden. Tip the spring onions onto a small baking tray, drizzle with the oil and season. Roast these alongside the pastry for the final 5 mins of cooking time.
Meanwhile, cook the pancetta in a small frying pan set over a medium heat until golden. Remove from the heat and set aside to cool.
Mix the soft cheese, chopped herbs, egg yolk, half the parmesan and the mustard together and season well. Gently squash the middle of the tart base down using the back of a spoon, then spread over the soft cheese mixture. Scatter the defrosted peas and pancetta over the soft cheese mixture, then arrange the roasted spring onions on top. Sprinkle over the remaining parmesan and return to the oven for 10 mins. Scatter with the remaining whole-leaf herbs, then slice and serve.