Prepare the chicken: Season the chicken breasts with salt and pepper on both sides.
Sear the chicken: In a large skillet over medium-high heat, heat the olive oil. Add the chicken breasts and sear for about 6-7 minutes per side until golden brown and cooked through. Once done, remove the chicken from the skillet and set aside.
Sauté the vegetables: In the same skillet, add the chopped onion and sauté for 3-4 minutes until translucent. Add the minced garlic and sliced mushrooms, cooking until the mushrooms are soft and browned, about 5-7 minutes.
Create the sauce: Pour in the chicken broth and bring to a simmer. Let it cook for 3-4 minutes, then add the heavy cream and thyme, stirring well to combine. Allow the mixture to simmer for another 5 minutes until slightly thickened.
Combine and heat through: Place the chicken breasts back into the skillet, coating them with the creamy mushroom sauce. Let them simmer in the sauce for an additional 2-3 minutes to heat through and absorb flavor.
Serve: Transfer the chicken to a serving platter and spoon the creamy mushroom sauce on top. Garnish with chopped parsley before serving.