01 - Season both sides of the chicken breasts with salt and black pepper.
02 - Heat olive oil in a large skillet over medium-high heat. Add chicken breasts and sear for 4 to 5 minutes per side until golden brown and nearly cooked through. Remove and set aside on a plate.
03 - In the same skillet, add sliced mushrooms and sauté for 4 to 5 minutes until browned and their moisture evaporates.
04 - Add chopped onion and minced garlic to the skillet, cooking for 2 to 3 minutes until softened and fragrant.
05 - Stir in chicken broth, scraping up any browned bits from the pan. Bring to a simmer.
06 - Add heavy cream, Dijon mustard, dried thyme, and Parmesan cheese if using. Stir thoroughly and simmer for 2 to 3 minutes until the sauce slightly thickens.
07 - Return chicken breasts to the skillet, nestling them into the sauce. Simmer for 5 to 7 minutes until chicken is cooked through and the sauce is creamy.
08 - Sprinkle fresh parsley over the dish before serving.