Slice chicken breasts so that each is a thin cutlet; making sure that all cutlets are as equal in size as possible.
Sprinkle each side of the chicken cutlets with salt and pepper.
Coat lightly with flour and shake off excess flour.
Melt butter in a large skillet on medium-high heat.
Add onions, chicken, and garlic. Saute until a golden crust develops . . . about 5 minutes on each side if thickly cut, or 3 minutes on each side if thinly cut.
Add the onions and garlic.
Add the chicken broth/stock, and lemon juice. This will deglaze the flavor bits on the bottom of the pan to blend into the cream sauce.
Lower the flame heat to low and add the cream and the mustard.
Add Parmesan cheese.
Simmer for a few minutes to bring up the heat of the sauce.
Sprinkle with minced parsley.
Garnish with lemon slices and sprigs of Italian parsley.
Serve with pasta, mashed potatoes, or rice.