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Heat the butter in a shallow pan and gently cook the leeks with the thyme for 8 mins, or until soft. Add both types of mushrooms and cook for 5 mins more, then turn up the heat a little, add the chicken and stir until the chicken starts to brown. Scatter over the flour and stir to make a paste and continue to cook for 2 mins.
Splash in the wine, if using, and sizzle for 1 min, stirring, then pour in the stock, cream and lemon juice, season and simmer for 10 mins until the chicken is cooked through. Leave to cool, then pop in the fridge to chill. Can be made up to two days ahead. Assemble the pies as directed in Tom's pie pastry recipe, decorating the tops with any trimmings, if you like.