01 - Gently pat the chicken breasts dry with paper towels, then season both sides generously with salt and freshly ground black pepper.
02 - In a large skillet, heat the olive oil over medium-high heat. Carefully place the seasoned chicken breasts in the hot pan 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 for 5 to 6 minutes, stirring occasionally, until the mushrooms are browned and the onion has softened. Add the minced garlic and cook for 1 minute more, until fragrant.
04 - Pour in the chicken broth, scraping the bottom of the pan with a wooden spoon to loosen any flavorful browned bits. Bring the broth to a gentle simmer and cook for 2 minutes.
05 - Stir in the heavy cream, Dijon mustard, and dried thyme. Bring the sauce to a gentle simmer and cook for 3 to 4 minutes, stirring occasionally, until it begins to thicken slightly.
06 - Return the seared chicken breasts to the skillet, nestling them into the creamy mushroom sauce. Allow to simmer for 2 to 3 minutes to reheat the chicken and allow the flavors to meld. Sprinkle with fresh chopped parsley before serving immediately.