Preheat the oven to 180℃.
In a large saucepan, heat the butter with a tsp of oil. Fry the onion and the thyme over a medium heat for about 7-8 minutes, until the onions are soft.
Add the plain flour, and stir. Cook for a further minute.
Pour in the chicken stock and stir well.
Cut the chicken fillets into bite size chunks. I used chicken thighs, but you can use breast if you prefer.
Add the chicken to the saucepan. Bring it to the boil, then turn down to simmer for ten minutes until it has thickened.
Meanwhile, in a large frying pan, melt some butter with a tsp of oil. Fry the mushrooms over a high heat for 2-3 minutes, stirring often.
Add the mushrooms to the saucepan, then stir in the créme fraîche and dijon mustard. Add the spinach leaves and stir well.
Taste for seasoning, it will need a little salt and pepper.
Use a large ladle to transfer the filling into a 24 cm pie dish.
Use a pastry brush to brush a little water around the edge of the dish. Cut a few 2 cm wide strips of pastry and line the lip of the pie dish.
Lay the remaining pastry across the top of the pie, pressing down around the edges. Use a sharp knife to cut away any excess pastry.
Crimp the edges by using your thumb and forefinger of one hand and the forefinger of the other.
Cut two slits in the middle of the pastry to allow steam to escape. Poke two sprigs of rosemary into the slits.
Lightly beat the egg and brush it all over the pastry.
Place the pie into the oven for about 25-30 minutes until it has a puffed up and golden top. When it looks done, it is done.
