Pre-heat the oven to 190°C.
Roll the pastry out on a lightly floured surface to 1cm thickness. Line a 7 inch straight-sided tart tin with the pastry, cutting off the overhanging pastry (of which there should be a lot). Gather the excess pastry into a ball and roll out to a 7 inch round of pastry and reserve to one side. Prick the base of the pastry and chill until needed.
Fry the lardons in a large, heavy-based saucepan set over a medium heat until browned. Transfer to a plate lined with kitchen paper to absorb the excess fat. Add the mushrooms to the saucepan and saute with a little salt until soft and broken down; 10-15 minutes, stirring occasionally. Remove from the heat and stir in the parsley and pancetta, adjusting the seasoning if necessary.
Fill the pastry with nonstick baking paper and baking beans and blind bake for 10-15 minutes unitil the edges are lightly coloured. Remove and discard the baking paper and baking beans. Bake for a further 4-5 minutes until the base is cooked.
Remove and brush all over with some of the beaten egg before returning to the oven for 2 minutes. Remove and fill with the mushroom mixture. Cover with the prepared 7 inch round of pastry and brush all over with the remaining beaten egg. Prick in a few places near the centre and return to the oven for 12-15 minutes until the top is cooked and browned.
Remove from the oven when ready and transfer to a wire rack to cool a little. Remove from the tin once cool enough to handle, then slice into portions and serve on a plate.
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