Cook the Chicken: Heat olive oil in a skillet over medium-high heat. Add the sliced chicken breasts and cook until browned on both sides and fully cooked through (about 6–8 minutes). Remove from the skillet and set aside.

Sauté the Vegetables: In the same skillet, add diced onion and minced garlic. Sauté for about 3 minutes until softened and translucent.

Cook the Mushrooms: Add sliced mushrooms to the skillet and cook until browned and softened, about 5–7 minutes.

Make the Sauce: Sprinkle flour over the mushrooms and stir to coat evenly. Gradually pour in the chicken broth, whisking to prevent lumps. Bring to a simmer and cook for 3–5 minutes, allowing the sauce to thicken.

Add the Cream: Stir in Dijon mustard and sour cream. Mix well until the sauce is smooth and creamy.

Combine the Chicken and Sauce: Return the cooked chicken to the skillet and stir to coat it in the sauce. Heat everything through for 2–3 minutes.

Season and Serve: Season with salt and pepper to taste. Serve hot, garnished with freshly chopped parsley.