01 - Pat the chicken thighs dry and season both sides evenly with salt and black pepper.
02 - Heat olive oil in a large skillet over medium-high heat. Place chicken thighs skin-side down and sear until golden brown, about 5-6 minutes, then flip and cook 3-4 minutes more. Remove from skillet and set aside.
03 - Reduce heat to medium. Add butter, mushrooms, and chopped onion to the skillet. Sauté until mushrooms turn golden and onions soften, approximately 5-6 minutes.
04 - Stir in minced garlic and cook for 1 minute until fragrant.
05 - Pour in chicken broth, scraping up any browned bits from the skillet. Simmer for 2 minutes to concentrate flavors.
06 - Add heavy cream, Dijon mustard, thyme, oregano, and red pepper flakes. Stir to blend thoroughly and bring to a gentle simmer.
07 - Return chicken thighs to the skillet, skin-side up. Spoon sauce over them, cover, and simmer gently for 15-20 minutes until internal temperature reaches 165°F (75°C).
08 - Remove the lid and simmer uncovered for 2-3 minutes to slightly thicken the sauce.
09 - Sprinkle chopped parsley and optional chives over the dish before plating.