01 - Season both sides of chicken breasts evenly with salt and black pepper.
02 - Heat olive oil in a large skillet over medium-high heat. Sear chicken breasts until golden brown on both sides, about 4 to 5 minutes per side. Remove and set aside.
03 - In the same skillet, sauté chopped onion for 2 to 3 minutes until softened. Add minced garlic and sliced mushrooms; cook until mushrooms are browned and moisture evaporates, approximately 5 minutes.
04 - Add chicken broth to skillet, scraping brown bits from the bottom. Stir in heavy cream, thyme, Dijon mustard, and Parmesan cheese if using.
05 - Return seared chicken breasts to skillet. Cover and simmer over low heat for 10 to 12 minutes until chicken is cooked through and sauce has thickened.
06 - Remove from heat, sprinkle with chopped parsley, and serve hot alongside your choice of rice, pasta, or crusty bread.