To joint the birds, bend the legs and thighs away from the birds, then cut between the joint to separate them. Cut between the thigh and leg joints to make two pieces. Bend back the wings and cut them off. Set aside the wings for the stock.
Cut either side of the breast bone and remove the breasts. Pull the skin off all the pieces of meat. Place the carcasses, skin and the wings in a pan with 900ml/1½ pints of water and bring to the boil to make stock. Simmer for 30 minutes, then remove from the heat.
Set the oven to 180C/350F/Gas 4 (160C fan). Place the porcini mushrooms in a small bowl, cover with four tablespoons of boiling water and set aside to plump up.
Heat the oil in a large frying pan and fry the guinea fowl pieces for about five minutes, or until golden-brown on all sides. You may need to do this in two batches. Remove the meat with a slotted spoon and place in a large roasting tin. Don't wash the frying pan, you will need it later.
Roast the guinea fowl in the oven for 25 minutes and then remove the guinea fowl breasts. Return the legs and thighs to the oven to cook for a further 15 minutes. Cut the breasts diagonally in half to give two portions. Set the meat aside once cooked and keep it warm for serving.
Add the lardons and shallots to the frying pan and fry gently for 10 minutes, or until the shallots are tender, but not browned. Add the thyme and sherry (or wine) and boil for a few minutes, or until the volume of liquid has reduced by a third.
Melt the butter in a small pan, add the flour and cook for a minute, or until thickened. Gradually pour in 500ml/18fl oz of the stock, stirring until thickened. Pour the sauce into the pan with the lardons and onions.
Strain the soaked mushrooms through a sieve, reserving the water. Chop the mushrooms finely and add them to the sauce with the soaking water. Add the fresh mushrooms.
Cook for five minutes, or until the mushrooms are tender. Season to taste. Stir in the crème fraiche, sprinkle with parsley. Serve the guinea fowl with the sauce alongside.