Pre-heat the oven to 200°c. Cut the chicken into chunks. Put a lug of olive oil and butter into a hot, large pan. Add the chicken and cook for a few minutes until sealed.
Meanwhile, make the stock and then wash and trim the spring onions and slice. Grate the button mushrooms before adding to the pan with the onions. Leave for 2 minutes before adding 1 heaped tbsps of plain flour, crème fraîche and mustard. Stir well.
Add the chicken stock and stir again. Add the herbs and the nutmeg, and season. Leave to simmer for 5-10 minutes or so.
Pour the mixture into an ovenproof dish. Roll out the pastry and place on top, tidying up the edges. Beat the egg and brush a little over the pastry.
Place in the oven and cook for 15-20 minutes, or until the pastry is golden. Serve with some steamed vegetables and mash.
