In the same pan, where you cooked chicken, heat another tablespoon of olive oil, then add sliced mushrooms, salt and pepper and saute for 5 minutes over medium heat, then push them to a side and add the chopped onions. Cook them over low heat for 2-3 minutes, then add 2 tbsp of flour, stir and cook for a minute longer, then add the marsala wine and turn the heat up to medium, add chicken stock and bring to a simmer, then reduce the heat to a gentle simmer and return the chicken to the pan. Pour in the cream and let it heat through in the sauce, simmer for a few minutes to allow the sauce to thicken to a desired consistency.