01 - Pat the chicken breasts dry with paper towels. Season both sides generously with salt, black 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 chicken to a plate and set aside.
03 - Reduce heat to medium. Melt butter in the same skillet, then 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 completely.
05 - Stir in minced garlic and cook for 30 seconds until fragrant, being careful not to burn it.
06 - Pour in chicken broth and Dijon mustard, using a wooden spoon to scrape up any browned bits from the bottom of the pan. Let simmer for 2 minutes to reduce slightly.
07 - Stir in heavy cream and fresh thyme leaves. Bring to a gentle simmer, stirring constantly to combine flavors.
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 (internal temperature reaches 165°F) and sauce has thickened.
09 - Remove from heat. Garnish with chopped fresh parsley and optional grated Parmesan cheese. Serve immediately with mashed potatoes, rice, or crusty bread.