Preheat oven to 200ºC (180ºC fan oven)/400ºF/Gas 6.
Dry fry the pancetta in a heavy based frying pan, once just cooked remove from pan with a slotted spoon and keep to one side.
Add the butter and oil to the pan, heat and then add the peeled shallots. Cook until golden on outside and tender, turning frequently (approx. it will take 10 minutes). When the onions are nearly ready, add the mushrooms and cook for a few minutes longer. Season, combine with the pancetta and allow to cool completely.
Roll out the pastry large enough to cut out a 23-25 cm/9-10” disc. Place on a lined baking sheet.
Pile the onion mixture evenly over the pastry (you may have to halve the shallots if they are big) leaving a 2.5 cm/1” border all round. Sprinkle with a little chopped parsley, brush border with beaten egg and bake for approx. 20 minutes, until pastry is puffed and golden. Serve immediately .