Put breasts on cutting board, lay plastic wrap over and pound until 1/4 inch thick.
Put flour, salt & pepper in shallow dish; mix.
Heat oil over medium heat in large skillet. When oil is hot, dredge both sides of chicken in flour mix, shaking off excess. Put in pan and fry for 5 minutes each side until golden. Remove chicken to plate.
Lower heat and add prosciutto/pancetta to drippings left in pan, sauté for 1 minute.
Add onion and garlic, sauté 2 minutes.
Add mushrooms and sauté until nicely browned, about 5 minutes. Add pepper to taste.
Pour Marsala/sherry in pan and boil for a few seconds to evaporate alcohol. Add chicken stock and simmer for 1-2 minutes to reduce sauce.
Stir in butter and cream, return chicken to pan, simmer for 1 minute to reheat chicken.
Serve over your choice of mashed potatoes or pasta.