Roast chicken stuffed with girolles, new potatoes and spring onions

This chicken has a rich stuffing of fresh duck foie gras, bacon, and chicken liver, as well as a second herb stuffing under the skin, of mixed herb butter that permeates the meat while cooking, giving it a wonderful flavour.
Serves 4
2 x 3lb free-range chickens
For the chicken glaze:
2 tbs chicken fat
(reserved from the chickens)
2 small garlic cloves, peeled
1 small onion, chopped
1 sprig fresh thyme
about 2 cups chicken stock or water
For the herb butter:
10 tbs unsalted butter, softened
1/2 bunch each: fresh parsley, chervil and tarragon, stems removed, leaves chopped
1/2 bunch fresh chives, chopped
fine sea salt and ground black pepper
For the stuffing:
60g slab bacon or pancetta, diced
85g raw duck foie gras, cut into cubes
fine sea salt and ground black pepper
40g chicken liver (reserved from the chickens), cut into1.5cm cubes
85g white sandwich bread crusts removed and cut into 1.5 cm cubes
For the vegetables:
3 tbs olive oil or goose fat
575g small new potatoes, scrubbed
coarse sea salt
575g girolle mushrooms, trimmed and cleaned
4 tbs unsalted butter
1 small shallot, finely chopped
1 sprig fresh thyme
225g pearl onions, peeled
1 tbs sugar
fine sea salt and ground black pepper
Pull out and reserve the two pockets of fat from inside the body cavity of each chicken; reserve two tablespoons of the fat for the chicken glaze. Cut off the wing tips, coarsely chop the chicken fat. Place it in a small saucepan and heat over medium heat until it starts to melt. Add a garlic clove and the chopped wing tips and necks and cook, stirring often, until browned - about 10 minutes. Pour off any excess fat and stir in the onion, thyme and the remaining garlic. Pour in enough chicken stock to barely cover. Bring to a boil, lower the heat, and simmer until syrupy and reduced to about 1/3 cup. Strain into a small saucepan, add a drop of water, and set aside.
Meanwhile, make the herb butter: In a medium bowl, mix the softened butter and chopped herbs until well blended. Season to taste with salt and pepper. Place the herb butter on a piece of plastic wrap and spread it evenly about .75 cm thick. Place in the freezer until firm.
To prepare the stuffing:
Heat a medium non-stick skillet over medium-high heat. Add the foie gras cubes and sauté until lightly browned - about 30 seconds. Season with salt and pepper. Transfer the foie gras, with its fat, to a fine sieve set over a small bowl to drain; reserve the fat in the bowl. Add the bacon to the skillet and cook, stirring frequently, until it begins to render its fat. With the slotted spoon, transfer to a small bowl. Add the chicken livers to the fat remaining in the skillet and cook, stirring, until seared on all sides - about 1 minute. Season with salt and pepper. Remove with the spoon and add to the bacon. Pour off the fat. Add the foie gras to the bacon and add the reserved foie gras fat to the skillet. Heat over medium-high heat, add the bread cubes, and cook, tossing and stirring often, until golden - about 4 minutes. Add to the bowl and mix all the ingredients together.
To roast the chicken:
Preheat the oven to 180C/Gas 4. Cut the chilled herb butter into 5-7.5cm-wide sheets. Oil your hands, and very gently loosen the skin away from the flesh. Slide the herb butter sheets under the skin and then massage so that the butter softens and evens out.
Divide the stuffing between the chickens and truss them. Season with salt and pepper and place in a roasting pan. Roast, basting often, for about 1 hour and 10 minutes, until the skin is golden brown and the juices run clear. Transfer to a wire rack and rest for 10 minutes. Skim off any fat from the juices and add the juices to the chicken glaze.
While the chicken is roasting, prepare the vegetables: Heat 2 tbs of the oil in a large skillet. Add the new potatoes, stir to coat with the oil, and season with coarse salt. Cover and cook over medium heat, shaking the pan often, until the potatoes are golden brown and tender - 30-35 minutes.
Meanwhile, in another skillet, heat the remaining oil over high heat. Add the mushrooms and sauté, stirring often, until tender - about 10 minutes. Add 1 tbs of the butter and the shallot and cook for 1 minute. Add the girolles to the cooked potatoes, add the thyme and toss to mix. Set aside.
In a small skillet, combine the pearl onions with 2 tbs of the butter and the sugar. Add enough water to just cover and season with a pinch of salt. Bring to a boil, then reduce the heat to low and simmer until the liquid has reduced to a golden brown glaze and the onions are tender - 8 to 10 minutes. Roll the onions around in the pan to coat with the glaze; add the potatoes.
To serve, add the remaining 1 tbs butter and the reserved chicken glaze to the vegetables and reheat over low heat until the liquid is simmering. Place the chickens on a large serving platter. Surround with the vegetables and serve.