01 - Pat the chicken thighs completely dry using paper towels. Generously season both sides of each thigh with salt and black pepper, ensuring even coverage.
02 - Heat olive oil in a large skillet over medium-high heat until shimmering. Place chicken thighs in the pan and sear for 4-5 minutes per side until a deep golden-brown crust forms. Transfer chicken to a plate and set aside.
03 - Reduce heat to medium. Add butter to the same skillet, allowing it to melt and coat the bottom. Add sliced mushrooms and chopped onion, sautéing for 5-6 minutes until vegetables are softened and golden brown.
04 - Stir in minced garlic and cook for 1 minute until fragrant, being careful not to burn it.
05 - Pour in chicken broth, using a wooden spoon to scrape up any browned bits from the bottom of the pan. Let the mixture simmer for 2 minutes to reduce slightly.
06 - Add heavy cream, Dijon mustard, dried thyme, and red pepper flakes if desired. Stir thoroughly to combine all ingredients and allow the sauce to begin thickening.
07 - Return the seared chicken thighs to the skillet, spooning some sauce over each piece. Simmer uncovered for 15 minutes until chicken is fully cooked through and the sauce has reached a creamy, coating consistency.
08 - Stir in grated Parmesan cheese and half of the chopped parsley until cheese is melted and incorporated. Taste the sauce and adjust salt and pepper as needed.
09 - Plate the chicken thighs with generous amounts of sauce. Garnish with remaining fresh parsley and serve immediately while hot.