Season both sides of the chicken breasts with salt, pepper, and garlic powder. Heat olive oil in a large skillet over medium-high heat. Add the chicken and sear for 4–5 minutes per side until golden and mostly cooked through. Remove from skillet and set aside.
In the same skillet, melt the butter. Add the sliced mushrooms and cook for 5–7 minutes until browned and tender. Stir in the garlic and thyme, and cook for another 1–2 minutes until fragrant.
Pour in the chicken broth and scrape up any browned bits from the bottom of the pan. Let it simmer for 2–3 minutes. Lower the heat and stir in the heavy cream and Parmesan, if using. Simmer gently until the sauce thickens slightly, about 3–4 minutes.
Add the chicken back into the skillet and spoon some of the sauce over the top. Cover and cook for another 5–7 minutes until the chicken is fully cooked and the sauce has thickened.
Garnish with fresh parsley and serve hot over mashed potatoes, pasta, or rice.