1Heat the olive oil and butter in a large pan over medium heat. Add the diced chicken thighs and cook for 4–5 minutes until lightly golden. Remove from the pan and set aside.
2In the same pan, cook the mushrooms until browned, then add the onions and a knob of butter and cook for 5 minutes until translucent. Add the garlic and thyme until fragrant.
3Return the chicken to the pan, sprinkle in the flour, stir well, and cook for 1 minute.
4Pour in the white wine (if using) and allow to reduce for 2 minutes.
5Add the chicken stock, stir well, and simmer for 5–8 minutes until the sauce thickens.
6Stir in the double cream and Dijon mustard. Season with salt and pepper, then simmer gently until the sauce is thick.
7Place the pastry into a springform tin, spoon in the mixture, add the lid, crimp to seal, brush with beaten egg, and bake at 200°C for 30-35 minutes.