Cook the sliced onion in a large frying pan until soft. Add the diced chicken and cook until lightly coloured; drain excess fat.
Add the white wine followed by the sliced mushrooms. Reduce the heat and simmer.
White Sauce: Melt the margarine in a saucepan then add the flour and mix. Cook for a few minutes over a gentle heat to make a roux. Gradually add the milk stirring in a little at a time until you get a smooth thick sauce.
Add the white sauce to the chicken and stir well.
Add the diced parsley to serve.
Serve with new potatoes and seasonal vegetables.