Sit the chicken in a large, lidded saucepan. Pour over the milk and stock, then nestle in 1 onion half and the bay leaves. Bring to a simmer over a medium heat, then cover and cook gently for 1 hr 15 mins until the chicken is cooked through – when ready, it should be opaque through the middle and the internal temperature should read 70C on a meat thermometer. Remove from the heat and leave the chicken to cool in the poaching liquid for 30mins, or until cool enough to handle. Drain, reserving 600ml of the poaching liquid, then shred the meat off the carcass using two forks. Discard the bones (or see tip, below).

Finely chop the remaining onion half. Heat a deep frying pan over a medium heat and fry the pancetta for 5 mins until crisp. Add the chopped onion and fry for 10 mins more until the onion has softened. Transfer the mixture to a plate using a slotted spoon and set aside.

Melt the butter in the same pan, then stir in the flour and cook for 2 mins. Pour in the wine and bring to the boil, stirring continuously. Remove from the heat and slowly whisk in the reserved poaching liquid, a little at a time. Return the pan to a medium heat and cook for 5 mins, whisking continuously until thickened. Stir through the shredded chicken, the mustard, double cream and the fried pancetta and onion. Season to taste. Spoon the mixture into a large ovenproof dish.

Heat the oven to 200C/180C fan/ gas 6. To make the topping, cook the potatoes in a pan of boiling salted water for 15 mins until easily pierced with a cutlery knife. Drain, then tip back into the pan and leave to steam-dry for 10 mins. Melt the butter in a frying pan over a medium heat and fry the spring onions for 5 mins. Mash the potatoes with a potato masher or fork, then stir through the buttery spring onions, the milk and seasoning. Spoon the mash over the chicken filling, then scatter over the cheese and bake for 30-35 mins until the topping is golden and crisp. The unbaked pie can be kept frozen for up to a month. Defrost thoroughly in the fridge overnight, then bake as above.