Cut the chicken breasts in half lengthwise. Place one chicken breast at a time in a heavy-duty freezer bag (unsealed) and use a tenderizing mallet (or a rolling pin…whatever is safe and sturdy that you have on hand) to hammer each one 1/4" thin, starting in the thicker middle part of the breasts and working your way to the ends. Repeat this process until all breast pieces are 1/4" thick and equally flat.
Combine ½ tsp salt, black pepper, dried Italian herb seasoning, and all-purpose flour in a shallow bowl, reserving 1 Tbsp of flour. Lightly coat the chicken breasts in the seasoning mixture and shake off excess flour. Transfer to a clean plate.
Heat vegetable oil in a large skillet over medium-high heat. Once hot, add the chicken breasts and cook on each side for 3-4 minutes. Try to not crowd your pan! You can do this in 2 batches if you need to. Once the chicken is cooked, transfer to a clean plate and cover with foil to keep warm.
Turn the heat down to medium. Add 1 tsp sea salt, sliced mushrooms, and onions. Spread them out on the pan, flipping as needed so each mushroom piece browns, about 3 minutes. Scrape the bottom of your pan with a wooden spoon as you go; the browned bits on the bottom become a tasty part of your sauce!
Add the Marsala wine and broth to the pan, scraping the browned bits off the bottom again. Simmer until the liquid reduces by 1/3, about 4 minutes on medium-high heat.
Now, make a slurry with your reserved 1 Tbsp of flour; whisk the flour with a small amount of water or liquid from your cooking pan until there are no lumps. Then, whisk in the flour “slurry”, minced parsley, and butter until it has completely melted. Finally, add the chicken cutlets back, flipping them halfway, cooking for another 4 minutes total. Let the sauce thicken while the chicken finishes cooking in the sauce.
Garnish with additional minced parsley (optional) and adjust salt and pepper to your liking. Enjoy over any pasta you have on hand!
¹ For a gluten-free chicken Marsala, use a 1:1 gluten-free flour as an equal swap for coating the chicken and thickening the sauce. I like King Arthur Gluten-Free Measure for Measure Flour because it browns well and behaves the most like all-purpose flour (in my experience!). You can also thicken the sauce with ½ Tbsp certified gluten-free cornstarch if preferred.² Any mushroom will work here, but I highly suggest using baby bella mushrooms because they have a rich flavor and add earthiness to the dish! ³ I use Marsala cooking wine to keep this Chicken marsala budget-friendly. For a more authentic flavor, use straight dry Marsala wine (the kind you'd drink), not sweet. See my "Best Wine for Chicken Marsala" note below for more details.⁴ Chicken broth forms the savory base of the sauce. I use Better than Bouillon because it's super budget-friendly and easy to keep in the fridge. Vegetable bouillon or broth is also fine.