Prepare the Chicken: Season the chicken breasts with salt, pepper, and thyme on both sides.

Cook the Chicken: Heat the olive oil or butter in a large skillet over medium-high heat. Once hot, add the chicken breasts and cook for about 6-7 minutes per side, or until golden brown and cooked through. Remove the chicken from the pan and set aside.

Sauté the Vegetables: In the same pan, add the onions and garlic. Sauté for about 2 minutes, or until softened and fragrant. Add the sliced mushrooms and cook for 5 minutes, stirring occasionally, until the mushrooms release their juices and start to brown.

Make the Sauce: Pour in the chicken broth and bring to a simmer, scraping up any bits from the pan. Stir in the heavy cream and Dijon mustard (if using). Let the sauce cook for another 3-5 minutes, allowing it to thicken slightly.

Combine Chicken and Sauce: Return the cooked chicken to the pan, spooning some of the sauce over the top. Let it simmer for an additional 5-7 minutes, until the chicken is heated through and coated in the creamy sauce.

Serve: Garnish with fresh parsley and serve immediately over rice, pasta, or mashed potatoes.