01 - Pat chicken breasts dry with paper towels. Season both sides generously with salt, pepper, and garlic powder, ensuring even coverage.
02 - Heat olive oil in a large skillet over medium-high heat until shimmering. Add chicken breasts and sear for 4–5 minutes per side until golden brown and nearly cooked through. Transfer to a plate and reserve.
03 - Reduce heat to medium. Melt butter in the same skillet, add diced onions, and cook for 2–3 minutes until softened and translucent.
04 - Add sliced mushrooms and sauté for 5–6 minutes, stirring occasionally, until browned and all liquid has evaporated.
05 - Stir in minced garlic and cook for 30 seconds until fragrant, being careful not to burn.
06 - Pour in chicken broth and Dijon mustard, scraping up browned bits from bottom of pan with wooden spoon. Simmer for 2 minutes to reduce slightly.
07 - Stir in heavy cream and thyme leaves. Bring to gentle simmer, stirring constantly to incorporate.
08 - Return chicken to skillet, nestling into sauce. Reduce heat to low, cover, and cook for 5–7 minutes until chicken reaches internal temperature of 165°F and sauce thickens.
09 - Remove from heat. Sprinkle with fresh chopped parsley and optional Parmesan cheese. Serve immediately.