Sprinkle the flour over the vegetables and cook, stirring, for 2 mins, or until foaming. Gradually add the stock, stirring constantly to prevent lumps from forming, and bring to a boil. Add the chicken and Dijon mustard, reduce the heat to medium-low and cook, at a very gentle simmer, for 5 mins, or until the chicken is cooked through and the sauce is thickened. Remove the pan from the heat, and stir the peas into the chicken mixture.