While the pastry is baking, melt the butter in a large frying pan over medium-high heat.
Add the mushrooms and cook for 6–8 minutes until their moisture has evaporated and they begin to brown.
Add the sliced shallots and cook for 2–3 minutes until softened.
Stir in the garlic and cook for 30 seconds until fragrant.
Transfer the mushroom mixture to a bowl and set aside.
Add the chicken thigh pieces to the same pan and cook for 4–5 minutes until lightly golden on the outside.
Return the mushroom mixture to the pan.
Pour in the 125 ml white wine and simmer until almost completely reduced.
Add the 125 ml thickened cream and simmer gently for 2–3 minutes until the sauce lightly coats the chicken and mushrooms.
Stir through the Dijon mustard.
Remove from the heat and stir through the 2 tbsp crème fraîche and 2 tbsp tarragon.
Season with salt and freshly ground black pepper to taste.