Preheat the oven to 190ºC/gas 5 and line a baking tray with greaseproof paper.
Roll out the pastry on a lightly floured surface to about 5mm thick. Using a small cutter or wine glass, about 6cm across, cut out 12 rounds (you may need to re-roll the pastry).
Place the circles on the baking tray and prick their centres several times with a fork. Beat the egg, then eggwash the circles. Bake on the top shelf of the oven for about 12 minutes until puffed up, crisp and golden.
Roughly chop or tear the mushrooms, peel and finely slice the garlic, and crumble the chilli (if using). Pick and roughly chop the parsley.
Heat a lug of oil in a large pan and cook the mushrooms, garlic and chilli (if using), until golden.
Off the heat, squeeze in the lemon juice, then add the crème fraîche. Season well and fold through the parsley. If the mushrooms are too dry, add a splash of water to loosen the sauce.
For the vol-au-vents, cut a very thin layer off the top of each circle and use your knife to carefully hollow out a hole in each one.
Fill with the creamy mushrooms and serve – you can pop the pastry lids back on, if you wish.