Place chicken in a plastic bag, and lightly tenderize it with the smooth side. You can also use a rolling pin. Season with a hefty pinch of salt and pepper on each side.
Heat butter and olive oil in a large pan over medium heat
Once butter bubbles, add the chicken breasts seasoned with salt and black pepper.
Let chicken cook for 3-5 minutes on each side or until a golden brown color is achieved.
Remove from pan and set aside on a plate.
In the same pan, add shallots, garlic, and extra olive oil if needed. Stir well and avoid caramelization. Add mushrooms and cook for five minutes while stirring frequently.
Add flour and stir until you no longer see the white of the flour.
Add stock, marsala wine, and milk. Stir well.
Taste the sauce and add salt and pepper to taste. It shouldn't be bland.
Return the chicken back to the pan. Cover with lid, and reduce heat to medium-low to finish cooking until internal temperature reaches 160 deg F. The temperature will continue to rise as the chicken rests.
Serve with roasted potatoes or mashed potatoes