Filet each breasts into 2 cutlets, then cover with plastic wrap and flatten with a meat pounder until thin. Add some flour to a shallow dish and dredge the chicken cutlets, shaking off the excess flour.
Add the olive oil and butter to the frying pan over high heat until it sizzles, do not let brown. Add half of the chicken and sauté until brown on both sides. Remove the cooked chicken to a separate dish and add the remaining chicken to the frying pan and finish cooking it. Remove the chicken and add it to the dish.
Saute the mushrooms in the frying pan for a few minutes, then add the garlic tossing to combine. Add the Marsala wine and simmer for 3 minutes, then stir in 4 - 5 tbsps. of flour.
Pour in the beef stock, then return the chicken to the frying pan seasoning with the salt and pepper. Simmer for several minutes until the chicken is hot and the sauce has thickened. Taste and adjust for seasoning.