01 - Season both sides of chicken breasts evenly with salt and pepper, pressing gently to adhere.
02 - Heat olive oil in a large skillet over medium-high heat until shimmering. Add chicken and sear for 5 to 6 minutes per side until golden brown and internal temperature reaches 165°F. Remove chicken and set aside on a plate, tent loosely with foil to rest.
03 - In the same skillet, add sliced mushrooms. Sauté for 5 minutes until browned and most moisture has evaporated, stirring occasionally.
04 - Add chopped onion to mushrooms and sauté for 2 to 3 minutes until translucent. Add minced garlic and thyme, stirring constantly for 1 minute until fragrant.
05 - Pour in chicken stock, using a wooden spoon to scrape up browned bits from bottom of skillet. Simmer for 2 to 3 minutes until liquid reduces slightly.
06 - Stir in heavy cream, grated Parmesan, and Dijon mustard. Bring to a gentle simmer and cook for 3 to 4 minutes, stirring occasionally, until sauce thickens enough to coat the back of a spoon.
07 - Return chicken to skillet, nestling pieces into the sauce. Simmer for 2 to 3 minutes until chicken is heated through. Sprinkle with fresh parsley, adjust seasoning if needed, and serve hot with extra sauce spooned over each portion.