01 - Pat chicken breasts dry and season evenly with salt and pepper.
02 - Heat olive oil in a large skillet over medium-high heat. Sear chicken breasts 4–5 minutes per side until golden and almost cooked. Transfer to a plate and cover loosely with foil.
03 - In the same skillet, melt butter and cook diced onions for 2 minutes until softened.
04 - Add sliced mushrooms to the skillet and sauté 4–5 minutes until browned and tender.
05 - Stir in minced garlic and cook for 1 minute until fragrant.
06 - Pour in chicken broth, scraping browned bits from the skillet. Simmer for 2 minutes.
07 - Reduce heat. Stir in heavy cream, Dijon mustard, and Parmesan cheese until sauce thickens slightly, about 2–3 minutes.
08 - Return chicken to skillet and spoon sauce over. Cover and simmer on low for 5–7 minutes until chicken reaches an internal temperature of 165°F.
09 - Stir in chopped parsley and taste to adjust seasoning if necessary.
10 - Serve immediately, garnished with additional parsley.