Season the chicken with salt and pepper, then dredge in flour, shaking off any excess.
In a large skillet, heat the olive oil over medium-high heat. Add the chicken and cook until golden and just cooked through, about 3-4 minutes per side. Remove to a plate and tent with foil.
In the same skillet, add the mushrooms. Sauté until they release their moisture and start to brown, about 5-7 minutes. Add the garlic and cook for another 30 seconds.
Pour in the Marsala wine, scraping up any browned bits from the bottom. Let it simmer for 2-3 minutes until slightly reduced.
Stir in the chicken stock and simmer until the sauce thickens a bit, about 4-5 minutes. Return the chicken to the pan, spooning some sauce over it. Stir in the butter until glossy.
Taste and adjust salt or pepper if needed. Garnish with parsley and serve over pasta, mashed potatoes, or a bed of spinach.