Preheat the oven to 200°C/gas mark 6. Place the flours in a bowl and add the butter. Using your fingertips, rub the butter into the flour until it resembles coarse breadcrumbs. Slowly add the milk 1 tbsp at a time, mixing with a knife until the mixture starts to come together to form a dough. Wrap in clingfilm and chill in the fridge while you prepare the pie filling.
Place the chicken stock, garlic and bay leaf in a saucepan and add the chicken thigh fillets. Bring to a simmer, cover and cook for 15 minutes until the chicken is just cooked. Strain the chicken stock into a jug and set aside.
Chop the chicken into bitesize chunks. In the same pan, heat the olive oil. Add the onion and cook for 5 minutes until softened. Add the mushrooms and thyme and cook for a further 5 minutes until the mushrooms have started to soften and are golden.
Sprinkle over the flour and slowly add the chicken cooking liquid a little at a time, stirring constantly. Add the crème fraîche and simmer for 5 minutes until the sauce thickens.
Add the chicken pieces to the sauce with the Dijon mustard and peas and remove from the heat. Tip the mixture into a pie dish measuring about 25cm diameter and 5cm deep.
Dust the work surface with a little flour and then start to roll out the pastry to fit the top of the pie dish. Brush the edge of the pie dish with the beaten egg, then carefully lay the pastry over the top of the filling and crimp the edges to seal and trim away any excess pastry. Make a few small holes in the middle of the pastry for the steam to escape, then brush lightly with beaten egg. Bake for 20-25 minutes until the pastry is golden.
TOP TIP: Any leftover scraps from your pastry are perfect for decorating. Simply cut out your desired letters, brush them with egg wash and pop on a baking tray. Bake them along with the pie for the last 15-20 minutes, until golden brown and puffed up.