Heat the olive oil in a large skillet over medium heat. Season the chicken breasts with salt and pepper, then add them to the skillet. Cook for about 5-6 minutes on each side, or until golden brown and cooked through. Remove the chicken from the skillet and set aside.
In the same skillet, add the sliced onions and cook for about 3-4 minutes until they start to soften. Add the sliced mushrooms and continue to cook for another 5 minutes, stirring occasionally, until the mushrooms are tender.
Stir in the minced garlic and cook for an additional 1 minute until fragrant.
Pour in the chicken broth and add the Dijon mustard and dried thyme. Stir well to combine and bring the mixture to a simmer. Let it cook for about 2-3 minutes to thicken slightly.
Return the chicken breasts to the skillet, spooning the sauce over them. Cook for another 2 minutes to heat through.
Serve the chicken topped with the mushroom and onion sauce, garnished with fresh parsley.
Notes
For a creamier sauce, stir in 1/4 cup of heavy cream after adding the chicken broth.
Serve over rice, quinoa, or mashed potatoes to soak up the delicious sauce.