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Melt 20g of the butter in a flameproof casserole dish over a medium heat. Season the chicken and add to the dish with the pancetta. Fry for 5 mins until the pancetta is crispy and the chicken browned and cooked through. Set aside in a bowl.
Lower the heat, add the remaining butter and, when it starts foaming, add the onions. Cook for 10 mins until softened, then stir through the flour and cook for another 2 mins. Remove from the heat and slowly whisk in the chicken stock. Return to the hob, bring to the boil and cook for 5 mins. Stir through the tarragon, mustard and cream, followed by the chicken and pancetta, and cook for another 5 mins until the sauce is thickened.
Heat the grill to high. Heat the mashed potato following pack instructions, then spoon on top of the chicken filling in the casserole dish. Smooth and swirl the potato over right to the edges, drizzle with a little olive oil and put under the grill for 8-10 mins or until the top is golden and bubbling.
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