Put the stock and bay leaf in a saucepan. Bring to a boil, and turn off the heat.
Melt the butter in a saute pan and fry the onions for 2 minutes. Add the mushrooms and cook until shrunken, dark and golden. Add the garlic and cook for 1 minute. Sprinkle over the flour and cook, stirring, for 1 minute. Add the wine and cook until it disappears. Pour over the stock, add the chicken and cook, stirring, until the sauce thickens, about 3 minutes. Stir in the creme fraiche and mustard. Season with salt and pepper. Stir in the parsley.
Spoon the chicken mixture into the vol au vent shells and serve.