01 - Preheat your oven to 400°F (200°C).
02 - In a large oven-safe skillet or casserole dish over medium heat, melt the butter. Add the chopped onion and minced garlic. Sauté for 2–3 minutes until softened and fragrant.
03 - Add the sliced mushrooms to the skillet. Cook for an additional 5 minutes, stirring occasionally, until they are browned and their moisture has evaporated.
04 - Introduce the chicken pieces to the skillet. Cook for 4–5 minutes, until they are just beginning to brown on all sides.
05 - Evenly sprinkle the flour over the chicken and mushroom mixture. Stir thoroughly to combine and cook for 1 minute to toast the flour.
06 - Pour in the chicken broth, using your spoon to scrape any browned bits from the bottom of the pan. Add the heavy cream, softened cream cheese, dried thyme, dried parsley, salt, and black pepper. Stir continuously until the sauce is smooth and has begun to thicken, which should take about 3–4 minutes.
07 - If your skillet is not oven-safe, carefully transfer the entire mixture to a greased casserole dish.
08 - Evenly distribute the grated Parmesan cheese over the top of the casserole, if using. Bake, uncovered, in the preheated oven for 20 minutes, or until the casserole is bubbling and has a golden-brown crust.
09 - Allow the casserole to rest for 5 minutes after removing it from the oven. Garnish with fresh chopped parsley before serving.