Heat butter and oil together in a large frying pan over medium heat. Add onion. Cook, stirring occasionally, for 5 minutes, or until soft. Add mushrooms. Cook, stirring occasionally for 5 minutes, or until softened. Sprinkle over flour. Cook, stirring for 1 minute.
Add stock, stirring constantly. Bring to boil. Simmer, uncovered over a medium-low heat, for 5 minutes. Stir in cream and mustard. Simmer, stirring occasionally, for a further 8-10 minutes, or until thickened slightly. Stir in parsley. Season.
TO SERVE:
We recommend serving with roast lamb and roasted vegetables. Roast Lamb Recipe