Poach the chicken (you can do this ahead, if you like). Put it breast-side down in a large pot with half the leek, bay, carrot and peppercorns and cover with cold water. Bring the water to the boil, then reduce the heat to a light simmer. Cover and poach for 45 mins. Carefully remove the chicken from the water (drain the cavity so you don’t scald yourself), remove the breasts from the carcass and return the carcass to the poaching liquid for a further 12 mins. Remove the chicken from the pot and leave to cool, then strip the meat off the bones. Boil the stock until reduced by at least half, then strain.

Heat oven to 200C/180C fan/gas 6. Heat the butter and olive oil in a heavy-bottomed frying pan. Fry the remaining leek, onion and garlic for 8 mins until softened and aromatic. Deglaze with the wine and cook off for a few mins, until most of the liquid has evaporated. Add the peas to the pan, along with the watercress and tarragon. Pour over 200ml of the stock, season, grate over the zest of the lemon and add a little of the lemon juice. Bring to the boil and cook for a further 3 mins until the peas are cooked. Purée the sauce using a food processor or stick blender, then stir in the soured cream. Taste and season with salt, pepper and lemon juice if needed. Add the chicken to the sauce.

Pour the pie filling into a 23-25cm pie dish and allow to cool a little. Cut a circle from the puff pastry sheet the same size as the pie dish and top with the pastry. Use a pie bird or cut a slit to let out steam. Crimp the edges with a fork, brush over the beaten egg and bake for 30 mins until the crust is puffed and golden. Serve with mashed potato and carrots, if you like.