Butterfly each chicken breast to create two lobes. Pound each until ¼”- ½” thick. Stuff each breast, lay down chicken, filling, and chicken.
Heat a large sauté pan over medium heat, and add olive oil.
Dredge chicken in flour, shaking off excess. Sauté until golden brown. Lay in a baking dish after browned, and bake 10-20 minutes.
Add onions to sauté pan, and stir to loosen chicken drippings. After 2 minutes, add mushrooms and sauté until onions are translucent.
Deglaze the pan with marsala wine, heat to a simmer and add heavy cream. Simmer on low until reduced by half.
Serve chicken topped with sauce and mashed potatoes as desired.