Preheat oven to 200c/180c Fan/Gas Mark 6
Wrap each chicken breast loosely in foil and place in the oven for 20 minutes.. remove foil and cook for a further 5 minutes without foil.
Meanwhile slice mushrooms and very finely dice onion and add to a pan with a little Frylight. Sauté for a few minutes over a high heat.
Add stock, crushed Garlic clove and Worcestershire sauce to the pan, cover with a lid and allow to simmer until the stock has almost reduced and the onion is soft.
Reduce the heat and add the yogurt with salt & pepper. Mix well and then serve with your cooked chicken.