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.
Recipe courtesy of Laura Calder