Goosnargh chicken with stuffed leg, girolles, leeks and pommes purée

Place a blender bowl and mixing bowl into the fridge or freezer to chill – these need to be cold when making the chicken mousse

Make a brine by placing the water, salt, sugar, thyme and peppercorns in a large saucepan. Bring to the boil until the salt and sugar dissolves, then set aside to cool in the fridge until under 5°C

Meanwhile, make the chicken mousse. Blend the chicken breast with a pinch of salt in the chilled blender bowl until smooth. Add the egg white and blend again, then add the single cream and blend once more until completely incorporated. Pass the mixture through a fine sieve then beat in the chilled double cream. At this point, you can wrap a small amount of the mousse in cling film and poach it for a few minutes in simmering water to check for seasoning and texture. Once happy, add the chopped chervil to the mousse and set aside in the fridge

Prepare the chicken by removing the legs, discarding the skin and boning them out flat (you can ask your butcher to do this for you). Separate the breasts and set aside, then chop up the wings and carcass (discard the backbone)

Cook the girolles by melting the butter in a sauté pan over a medium-high heat. If the girolles are large, cut into 0.5cm slices, but if they’re small they can be left whole. Add the mushrooms and cook until they begin to smell roasted (about 5 minutes). Add the shallots and cook for a further minute, then add the stock and reduce until almost entirely evaporated and the mushrooms are glazed. Allow to cool

When the brine has cooled, place the chicken breasts into it and leave for 30 minutes

Meanwhile, stuff the chicken legs. Lay one of the chicken legs on a piece of cling film still attached to the roll – the thigh should be to your left and the drumstick end to your right. Season with salt and spread the chicken mousse 1cm-thick along the centre of the leg, from left to right

Press a line of the cooled girolles down the centre of the mousse, then cover with another layer of mousse (any leftover mushrooms can be added to the sauce later). Lay the other leg on top, this time with the thigh to your right and the drumstick end to your left. Season with salt then roll into a tight cylinder using the cling film. Tie the ends so you’re left with a neat cylinder shape. Set aside in the fridge

After 30 minutes, remove the chicken breasts from the brine and rinse in clean water. Pat dry and set aside. Preheat an oven to 180°C/gas mark 4 and preheat a water bath to 65°C

Blend the dried mushrooms and tarragon to a fine powder. If you’re buying ready-dried mushrooms, you might need to dry them out further in a dehydrator or low oven to get them to a fine powder consistency

Dust the chicken breasts with the powder then wrap each one in cling film to form a pillow shape. Vacuum seal and set aside to cook in the water bath later

2 hours before you plan to serve the dish, place the stuffed legs in the water bath. Meanwhile, place the potatoes in the oven and bake for 1 hour

While the potatoes are cooking, prepare the sauce. Place the chicken carcass and wings on a tray and roast for 20 minutes. Meanwhile, heat the butter in a heavy-bottomed pan and add the shallots. Cook for a few minutes until soft but not coloured, then add the chestnut mushrooms and continue to cook until the water they release has reduced by half. Add the thyme, garlic and dried mushrooms, cook for a further 2 minutes then add the cream and crème fraiche. Deglaze the chicken carcass pan with a little water or wine, then add to the sauce. Bring to the boil and simmer for 20 minutes, then pass through a fine sieve and check for seasoning. Keep warm

After the potatoes have been in the oven for 1 hour, remove them. Add the chicken breasts to the water bath to cook for the remaining 1 hour with the stuffed legs

Push the hot potatoes through a fine mesh sieve, discarding the skin. Bring the cream to the boil in a pan with half of the butter, then remove from the heat and beat the potato flesh into the cream. Beat in the remaining butter, season with salt and finish with the vinegar. Transfer to a piping bag and keep warm

Prepare the leeks by melting the butter in a pan large enough to fit the leeks in a single layer. Once foaming add the leeks and season with salt. Just before the leeks begin to colour, add the chicken stock. Bring to the boil and then turn down to a simmer, continuing to cook until the leeks are tender. Increase the heat until the stock starts to glaze the leeks

After the chicken has been in the water bath for the correct amount of time, you are ready to serve. Add any remaining girolles and the vin jaune to the warm sauce. Unwrap the chicken breasts and legs

Pipe the warm potato onto 4 plates and top each with 4 glazed leeks garnished with the chive flowers. Slice the chicken breasts in half and brush with a little butter, then place one piece on top of each plate of leeks. Slice the leg into 4 equal pieces and divide between the plates, then spoon plenty of sauce into the centre