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 cook for 5 to 6 minutes per side, until golden and cooked through. Remove chicken to a plate and tent with foil.
03 - In the same skillet, add mushrooms and onions. Sauté for 5 to 6 minutes, until softened and lightly browned.
04 - Add garlic and cook for 1 minute until fragrant.
05 - Pour in chicken broth, scraping up any browned bits from the pan. Simmer for 2 to 3 minutes.
06 - Reduce heat to medium-low. Stir in heavy cream, Dijon mustard, thyme, nutmeg, and Parmesan cheese. Let simmer for 3 to 4 minutes, until slightly thickened.
07 - Return chicken and any juices to the skillet. Spoon sauce over chicken and simmer for 2 to 3 minutes until heated through.
08 - Garnish with fresh parsley before serving.