If you are using dried mushrooms, heat the stock and soak them for 10 mins, then remove them, strain the liquid and set it aside. If you’re using fresh mushrooms, clean them thoroughly before using. Set a couple of the nicest-looking morels aside to decorate the top of the pie, and halve the rest.
Heat half the butter in a skillet and fry the halved morels for 3-4 mins or until wilted. Scoop them onto a plate and set aside. Heat the remaining butter and gently cook the shallots in the pan with the thyme and bay. Once softened, stir in the flour and cook for 1 min or until you have a sandy paste.
Pour in the sherry and sizzle, then carefully stir in the strained soaking liquid (or 200ml chicken stock if you’ve used fresh morels), followed by the crème fraîche. Season well and bring the sauce to a gentle simmer. Add the chicken and poach in the sauce for 10 mins or until the chicken is just cooked through. Remove the bay, stir through the asparagus, tarragon and fried morels, then remove from the heat.
Heat oven to 220C/200C fan/gas 8. The pastry needs to sit on top of the ingredients, so if your pan is too deep, use a pie dish instead. Roll out the pastry on a floured surface to the thickness of a £1 coin, then cut the pastry to fit the pan or dish, and drape it over the pie mixture using a rolling pin to help you. Liberally brush with egg, season the pastry with flaky sea salt, and pop your reserved morels on the top. Bake for 20 mins or until the pastry has puffed and is a deep golden brown. Leave tor rest for 5 mins before serving straight from the pan.