In a small saucepan melt the butter, season the chicken joint with salt and pepper and cook it in the hot butter till golden on both sides (approximately 5 minutes), then remove it to a plate.
Now add the onion to the pan and cook that for about 5 minutes also, or until softened, then return the chicken joint and sprinkle the tarragon over. Pour in the wine, cover the pan with a tight lid, and simmer very gently for 30 minutes or until the chicken is tender - the length of the cooking time will depend on its size. After that remove the lid and turn the heat up.
Let the sauce bubble and reduce slightly for about a minute, then whisk in the butter and flour paste till the sauce has thickened a bit and then turn the heat down and add the halved grapes and stir in the cream. Allow them to heat through, then taste and check the seasoning, and serve the chicken with the sauce poured over.