01 - Pat the chicken breasts dry with paper towels. Season both sides generously with salt and black pepper.
02 - Heat the olive oil in a large skillet over medium-high heat. Add the seasoned chicken breasts and sear for 4 to 5 minutes per side, until deeply golden brown and cooked through. Transfer the seared chicken to a plate and cover loosely with foil to keep warm.
03 - Reduce the heat to medium in the same skillet. Add the sliced mushrooms and chopped onion. Sauté, stirring occasionally, for 5 to 6 minutes, until the mushrooms are browned and the onion is softened.
04 - Stir in the minced garlic and cook for 1 minute until fragrant.
05 - Pour in the chicken broth and use a wooden spoon to scrape up any browned bits from the bottom of the pan. Allow to simmer for 2 minutes.
06 - Stir in the heavy cream, Dijon mustard, and dried thyme. Bring the sauce to a gentle simmer. Cook for 3 to 4 minutes, stirring occasionally, until the sauce thickens slightly.
07 - Return the cooked chicken breasts to the skillet, nestling them into the rich cream sauce. Simmer for 2 to 3 minutes to reheat the chicken and allow the flavors to meld. Garnish with chopped fresh parsley before serving.