In a shallow dish or bowl, mix together the flour, salt, pepper and oregano. Coat chicken pieces in flour mixture.
In a large frypan, melt butter in the oil over medium heat. Place chicken in the pan, and lightly brown on one side. Turn over the chicken pieces, and add mushrooms. Pour in wine and sherry. Cover the frypan; simmer chicken 10 minutes, turning once, until no longer pink and juices run clear.
This recipe has fundamental errors that make me refuse to even try it.
1) There is no such thing as "cooking sherry". There is either drinkable sherry or undrinkable sherry - just like wine. Most savoury recipes are happy with a good dry sherry.
2) No recipe with Marsala in it should have sherry within a million kms. Marsala is there for its uniqueness (a lovely drink and flavouring) - why ruin it with even very good sherry? In particular, you should not use Marsala 'substitutes' (sticky, sweet Boronia, for example) - there aren't any. It must be imported from Sicily. - 07 Feb 2013