01 - Pat chicken breasts dry and evenly season both sides with salt and black pepper.
02 - Heat olive oil in a large skillet over medium-high heat and sear chicken breasts for 4–5 minutes per side until golden and almost cooked through. Remove and keep warm covered loosely with foil.
03 - In the same skillet, melt butter over medium heat, add diced onions, and cook for 2 minutes until softened.
04 - Add sliced mushrooms to the skillet and sauté for 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 any browned bits from the pan, and simmer for 2 minutes.
07 - Reduce heat to low, stir in heavy cream, Dijon mustard, and Parmesan cheese, cooking until sauce is smooth and slightly thickened, about 2–3 minutes.
08 - Return chicken to the skillet, spoon sauce over each piece, cover, and simmer on low for 5–7 minutes until chicken reaches an internal temperature of 165°F.
09 - Stir in chopped parsley, adjust seasoning as desired.
10 - Plate the chicken breasts and garnish with additional parsley. Serve immediately.