Wrap the chicken breasts with the bacon and secure each piece with a toothpick. Sprinkle with pepper and garlic powder. Heat a frying pan over medium high heat and place in the breasts. Fry about 10 minutes per side, until bacon is very crisp.
Preheat oven to 180 degrees C. In a baking dish dissolve the stock cube in the water and sherry. Place in the chicken breasts then bake in preheated oven until cooked through and juices run clear, about 15 minutes.
Discard all but 2 tablespoons of the rendered bacon fat then fry the mushrooms and onion until lightly browned. Season with salt, pepper and garlic powder. Stir in the sherry and cook until reduced. Serve over chicken breasts.