Put the onions and 30g/1oz of the butter in a small pan and cover with cold water. Add a pinch of salt and cook over a medium heat for 15 minutes. Remove from the heat and set aside, leaving the onions in their cooking water.
Preheat the oven to 200C/400F/Gas 6.
Melt the remaining butter over a very gentle heat and bring slowly to the boil. Skim off the froth from the surface. Carefully pour the liquid butter into a bowl, holding back the milky sediment in the bottom of the pan. The clarified butter should be the colour of light olive oil.
Cook the broccoli stems in boiling salted water for two minutes, the refresh in cold water and drain. Cover with cling film and set aside.
Trim the chanterelle stalks, then use a damp tea towel to gently wipe away any traces of sand or earth.
Heat three tablespoons of the clarified butter in a frying pan over a medium-high heat, add the chanterelles and a pinch of salt and cook gently for 3-4 minutes. As soon as they have released their liquid, transfer to a bowl and cover with cling film, perforated in several places.
Heat the remaining clarified butter in an ovenproof pan set over a moderately high heat. Sprinkle the guinea fowl pieces and carcass with salt and add to the pan. Colour on all sides, then transfer the pan to the oven and cook for 15 minutes.
Take the guinea fowl out of the pan and put in a dish, leaving the browned carcass pieces in the pan. Cover the dish with foil and set aside. Pour off the fat from the pan.
Add the shallots, thyme, bay leaves and tarragon stalks to the carcass pieces in the pan. Pour in the wine and turn the heat up to high.
Use a wooden spoon or spatula to scrape the cooked on parts from the bottom of the pan to deglaze it. Continue to cook until the volume of liquid is reduced by two-thirds.
Pour in the stock and cook to reduce the volume of liquid by half, then add the cream and cook over a gentle heat until the sauce is thick enough to lightly coat the back of a spoon. Strain through a chinois cloth or fine sieve into a large pan and season with salt and pepper to taste.
Add the pieces of guinea fowl to the sauce, along with the onions, broccoli and chanterelles, making sure each is well drained. Bring to the barest simmer over a gentle heat, then add the lemon juice and two-thirds of the tarragon leaves and immediately remove from the heat.
To serve, tip the contents of the pan into a shallow serving dish, sprinkle with the remaining tarragon leaves and serve, with bread to mop up the delicious juices.