01 - Mix olive oil, minced garlic, salt, and black pepper in a small bowl. Coat both sides of the chicken breasts with the marinade and set aside while preparing the other ingredients.
02 - Heat a large skillet over medium-high. Add the marinated chicken breasts and sear for 3 to 4 minutes per side until golden. Remove to a plate and keep warm.
03 - Reduce skillet to medium heat. Melt unsalted butter, then sauté the chopped onion for 2 to 3 minutes until softened.
04 - Pour in chicken broth, stirring and scraping up any browned bits from the skillet. Simmer for 2 minutes.
05 - Blend in heavy cream, Dijon mustard, and chopped fresh parsley. Simmer gently.
06 - Return chicken breasts to skillet and nestle them into the sauce. Cover and cook for 10 to 12 minutes or until chicken reaches an internal temperature of 165°F.
07 - Add lemon juice, stir gently, and adjust seasoning to taste. Serve hot, garnished with chopped parsley and optional lemon wedges.