To butterfly the chicken: Slice the chicken breast down the middle horizontally lengthways to make 2 thin breast pieces. Place the chicken flat on the cutting board and lay a piece of plastic wrap over them. Using a mallet gently pound the chicken so it is an even thickness throughout. Season both sides of the chicken with salt and pepper, then coat both sides well with flour.
Partially cook the chicken on both sides until golden brown about 2 -3 minutes per side then remove from the pan and set aside.
Melt the butter in the pan. Add the onion and mushrooms and season with salt and pepper.
Cook for 3-5 minutes or until vegetables have softened. Add in the flour and cook while stirring for 1 minute. Add the wine and simmer until it evaporates completely then add the beef broth and bring to a simmer. Add the chicken back into the pan and simmer for 10 - 15 minutes or until the chicken has reached an internal temperature of 160 degrees using a thermometer.
The gravy should be a thickened smooth consistency. Remove from the heat when the gravy has reached the desired thickness. Season with salt and pepper to taste and sprinkle with parsley to garnish serve with crusty bread.