Butterfly and lightly pound the chicken to an even ¼–½ inch thickness. Season with salt and pepper, then lightly dredge in flour. Shake off the excess.

Heat the olive oil in a large sauté pan over medium-high heat. Sear the chicken 3–4 minutes per side until golden brown. Remove and keep warm.

Reduce the heat to medium. Add the butter and mushrooms. Cook until the mushrooms are browned and their moisture has evaporated, about 5–7 minutes. Add the garlic and cook for 30 seconds.

Pour in the Marsala wine, scraping the browned bits from the bottom of the pan. Reduce the wine by half.

Add the chicken stock and thyme. Simmer 3–5 minutes. Stir in the heavy cream, if using, and continue simmering until the sauce lightly coats the back of a spoon.

Return the chicken to the pan and simmer gently for 3–4 minutes, or until fully cooked. Taste and adjust seasoning.

Serve over hot white rice. Spoon the mushroom Marsala sauce generously over the chicken and garnish with chopped parsley.