Preheat oven to 220°C/430°F (200°/390°F fan-forced).
In a large frypan, saute mushrooms with 1 tablespoon of butter on medium-high heat. Once mushrooms have turned golden brown and nutty (after about 7 minutes), add the remaining tablespoon of butter, garlic and onions. Cook for a further minute.
Push the vegetables to the rims of the pan to make room for chicken. Add chicken into the pan, season with salt and pepper. Seal and brown the chicken for about 2 minutes, turning once.
Add and stir through dijon mustard and flour. Add chicken stock and stir until sauce thickens and there are no flour lumps. About 2 minutes.
Reduce the heat to low. Add fresh thyme leaves (discard the woody stems) and heavy cream. Stir through, taste and adjust for salt and pepper.
Turn the heat off, mix in frozen peas. ¼
Transfer pie filling into a pie dish (I used a 24cm/8inch round pie dish). Cover the pie dish with just thawed puff pastry and loosely tuck in the sides.
Brush egg wash all over the puff pastry. With a sharp knife, pierce an 'X' in the centre of the pie to allow steam to escape when baking.
Bake on the middle shelf for 25 minutes or until golden.