Tip the flour into a large food bag. Add the chicken pieces, two at a time, and shake well to coat evenly. Heat 2 tbsp oil in a large pan, add a few pieces of chicken and fry on all sides until well browned. You'll need to do this in batches and you may need a touch more oil or fat. Remove the pieces to a large saucepan or flameproof casserole.
Add 1 tbsp oil to the pan, add the shallots and fry gently until softened, but not browned. Add the wine and allow to bubble until it is reduced a little. Add the stock, herb bundle, mustard if using and seasoning and bring to the boil. Pour over the chicken.
Cover the pan tightly and simmer for 45 mins until the chicken is tender and the sauce thickened and reduced.
Meanwhile, separate the garlic head into individual cloves and put in a small pan with water to cover. Bring to the boil, then cover and simmer for 20-25 mins until the garlic is tender. Drain and cool under running cold water. Peel the garlic.
Heat half the butter and a splash of oil in a frying pan. Add the mushrooms and cook quickly over a moderate heat until just softened. Tip into a bowl. Wipe out the pan and add the remaining butter and a splash of oil. Add the garlic and fry gently, shaking the pan until lightly browned. Season lightly.
Stir the mushrooms and thre cream into the chicken and bring to the boil. Simmer for 5 mins, then taste and add more seasoning and a little lemon juice if it needs it. Spoon the chicken and sauce onto a warm platter and scatter with garlic.