01 - Pat chicken breasts dry with paper towels and season both sides generously with salt and black pepper.
02 - Heat olive oil in a large skillet over medium-high heat. Add chicken breasts and cook for 5-6 minutes per side until golden brown and cooked through. Transfer to a plate and cover loosely with foil.
03 - Reduce heat to medium. In the same skillet, melt butter. Add sliced mushrooms and cook, stirring occasionally, for 5 minutes until softened and browned.
04 - Add finely chopped onion and minced garlic to the skillet. Cook for 2 minutes until fragrant and translucent.
05 - Pour in chicken broth and stir, scraping up any browned bits from the bottom of the skillet. Simmer for 3 minutes to reduce slightly.
06 - Add heavy cream, Dijon mustard, fresh thyme leaves, and grated Parmesan cheese. Stir well and simmer for 2-3 minutes until sauce thickens slightly.
07 - Return chicken to the skillet, nestling pieces into the sauce. Simmer for 3-4 minutes until heated through. Sprinkle with fresh chopped parsley before serving.