01 - Season the chicken breasts evenly on both sides with salt and black pepper.
02 - Heat olive oil in a large skillet over medium-high heat. Add chicken breasts and sear for 4 to 5 minutes per side until golden brown and fully cooked. Remove from skillet and keep warm loosely covered with foil.
03 - Add unsalted butter to the same skillet. Sauté finely chopped onion for 2 minutes until softened. Add sliced mushrooms and cook for 5 to 6 minutes until browned and moisture has evaporated.
04 - Stir in minced garlic and thyme leaves, cooking for 1 minute until fragrant.
05 - Pour in chicken broth, scraping up browned bits from the bottom of the pan. Let simmer for 2 to 3 minutes to slightly reduce.
06 - Stir in heavy cream and bring the mixture to a gentle simmer. Cook for 3 to 4 minutes until the sauce thickens slightly.
07 - Return the chicken breasts to the skillet, spoon sauce and mushrooms over them, and simmer for 2 to 3 minutes until heated through.
08 - Sprinkle chopped fresh parsley over the dish before serving.