01 - Pat chicken breasts dry with paper towels. Season both sides generously with salt, black pepper, and garlic powder.
02 - Heat olive oil in a large skillet over medium-high heat. Add chicken breasts and sear for 4–5 minutes per side until golden brown and nearly cooked through. Transfer to a plate and set aside.
03 - Reduce heat to medium. Melt butter in the same skillet and add diced onions. Cook for 2–3 minutes until softened and translucent.
04 - Add sliced mushrooms and sauté for 5–6 minutes, stirring occasionally, until mushrooms are browned and their 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. Use a wooden spoon to scrape up any browned bits from the bottom of the pan. Simmer for 2 minutes.
07 - Stir in heavy cream and thyme leaves. Bring to a gentle simmer, stirring constantly to combine.
08 - Return chicken breasts to the skillet, nestling them into the sauce. Reduce heat to low, cover, and cook for 5–7 minutes until chicken is cooked through and sauce has thickened slightly.
09 - Garnish with chopped fresh parsley and optional grated Parmesan cheese. Serve immediately with mashed potatoes, rice, or crusty bread.