Preheat oven to 170°C.
Place a pie dish (or baking tray) over a medium heat, add a splash of olive oil and butter and sauté onion, carrot and tomato till soft.
Add some extra butter, chicken and flour, cook until brown. Then, add mushrooms.
Add Dijon mustard, crème fraiche and chicken stock and stir well.
Add thyme, rosemary, nutmeg and then season with salt and pepper. Allow to simmer.
Take off the heat, sprinkle with tasty cheese, salt and pepper and cover pie dish with puff pastry.
Squeeze with your finger tips to tuck the puff pastry around the edge, brush with egg wash then bake for 15 minutes, or until golden and puffy.
Remove from the oven and serve.