Cut the chicken breasts in half lengthwise to butterfly, then cut in half. Place in a zip top bag to pound to even thickness. If your chicken breasts are smaller, there's no need to cut them in half, simply pound them to about 1/4" thickness. Season the chicken generously with salt and pepper.

Heat 2 tablespoons of the oil in a large sauté pan. Add the chicken and cook until golden brown and it releases easily from the pan, about 2-3 minutes. Turn and cook another 2-3 minutes on the other side until a meat thermometer inserted into the center registers 160 degrees. Remove from the pan and place on a plate, then cover loosely with foil.

Add the additional tablespoon of olive oil to the pan. Add the mushrooms and cook for 4-5 minutes until browned and the liquid has evaporated, and be sure to scrape up the brown bits from the bottom of the pan to ensure that they don't burn. Add the shallots and garlic and cook for 1-2 minutes more.

Add the chicken stock, marsala wine, heavy cream and thyme to the pan. Bring to a boil, then reduce the heat to a high simmer and cook until the sauce has reduced to about half and it coats a wooden spoon, about 10-15 minutes.

Season the sauce with salt and pepper to taste, then add the chicken back to the pan along with any accumulated juices and simmer on low heat until warmed through, about 1-2 minutes. Serve immediately and enjoy!