Simmer onions with 30g butter in water for 15 minutes, then set aside in liquid.
Preheat the oven to 200°C.
Heat 100g butter, skim foam, and pour off liquid to create clarified butter.
Boil broccoli for two minutes, refresh in cold water, and drain.
Clean chanterelles by trimming stalks and wiping with a damp towel.
Sauté chanterelles in 3 tablespoons of clarified butter for 4 minutes and set aside.
Brown guinea fowl pieces and carcass in clarified butter, then roast at 200°C for 15 minutes.
Remove fowl pieces to a dish, cover with foil, and discard fat from the pan.
Add shallots, herbs, and wine to the carcass in the pan over high heat.
Deglaze the pan and reduce the liquid by two-thirds.
Add stock, reduce by half, stir in cream until thickened, and strain.
Combine fowl, vegetables, lemon juice, and tarragon in the sauce and simmer gently.
Transfer to a dish, garnish with remaining tarragon, and serve.