01 - Season both sides of the chicken breasts with salt and pepper.
02 - Heat the olive oil in a large skillet over medium-high heat. Add the seasoned chicken breasts and sear for 4–5 minutes per side, until golden brown and cooked through. Remove the chicken from the skillet and set aside.
03 - In the same skillet, add the sliced mushrooms and chopped onion. Sauté for 5–6 minutes, until the mushrooms are browned and the onions have softened.
04 - Add the minced garlic to the skillet and cook for an additional 1 minute until fragrant.
05 - Pour in the chicken broth, using a spoon to scrape up any browned bits from the bottom of the skillet.
06 - Reduce the heat to medium-low. Stir in the heavy cream, Dijon mustard, dried thyme, and ground nutmeg. Allow the sauce to simmer gently for 2–3 minutes, until it begins to thicken slightly.
07 - Return the seared chicken breasts to the skillet, nestling them into the sauce. Sprinkle the grated Parmesan cheese over the top. Continue to simmer for 3–4 minutes, spooning the sauce over the chicken to ensure it heats through.
08 - Garnish the dish with fresh chopped parsley just before serving.