01 - Season both sides of the chicken breasts with salt and pepper.
02 - Heat olive oil in a large skillet over medium-high heat. Add chicken breasts and sear until golden brown on both sides, about 4 to 5 minutes per side. Transfer to a plate and set aside.
03 - In the same skillet, add onion and cook for 2 to 3 minutes until softened. Add garlic and mushrooms; cook until mushrooms are browned and their moisture has evaporated, about 5 minutes.
04 - Pour in chicken broth, scraping up any browned bits from the bottom. Stir in heavy cream, thyme, Dijon mustard, and Parmesan cheese if using.
05 - Return chicken to the 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 rice, pasta, or crusty bread.