01 - Pat the chicken thighs dry with paper towels and season both sides generously with salt and pepper.
02 - Heat olive oil in a large skillet over medium-high heat. Sear chicken thighs for 4-5 minutes per side until golden brown and crispy. Remove chicken from skillet and set aside on a plate.
03 - In the same skillet, melt butter over medium heat. Add mushrooms and onion, sauté for 5-6 minutes until softened and golden brown.
04 - Stir in the 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. Simmer for 2 minutes to reduce slightly.
06 - Add heavy cream, Dijon mustard, dried thyme, and red pepper flakes if using. Stir thoroughly to combine all ingredients.
07 - Return chicken thighs to the skillet. Simmer uncovered over medium-low heat for 15 minutes, or until chicken is cooked through and sauce has thickened enough to coat a spoon.
08 - Stir in Parmesan cheese and half the fresh parsley. Taste and adjust salt and pepper if needed.
09 - Plate chicken thighs with generous sauce and garnish with remaining fresh parsley.