Place the chicken breasts on a large cutting board and cover with plastic wrap. Use a flat meat mallet to pound the chicken breasts until they reach equal thickness.
Transfer chicken breasts to a large ziplock bag. Add the all-purpose flour, Italian seasoning, fresh chopped rosemary, salt, and pepper to the ziplock bag with the chicken, seal, and shake well to completely coat the chicken breasts.
Heat a large skillet over medium-high heat. Add 2 tablespoons of butter to the skillet. Swirl around the skillet to coat until fully melted. Work in batches to cook the pounded chicken, approximately 5 minutes on each side, until fully cooked and golden brown. Remove the cooked pieces to a plate. Repeat with any additional chicken pieces, adding more butter to the skillet before frying, as needed.
Remove all chicken from the skillet, lower heat to medium and add the diced onion. Sauté the onions for approximately 5-6 minutes, stirring frequently. Add the minced garlic and sauté for one minute more.
Increase heat to high and immediately add the sweet marsala wine. Allow the marsala wine to boil down for 1 minute before adding the chicken stock, pancetta, and mushrooms to the skillet. Allow everything to cook together for 5 minutes, stirring occasionally.
Add the chicken cutlets back to the skillet, and allow the chicken to reheat approximately 1 minute. Season the chicken with salt and pepper and sprinkle with fresh chopped parsley just before serving.