01 - Season both sides of the chicken breasts with salt and pepper. Lightly dust with flour, shaking off any excess.
02 - Heat olive oil and 1 tbsp butter in a large skillet over medium-high heat. Add chicken breasts and cook for 4-5 minutes per side, or until golden and cooked through. Transfer to a plate and cover loosely with foil.
03 - In the same skillet, add the remaining 1 tbsp butter. Sauté the onion for 2 minutes until translucent. Add garlic and cook for 30 seconds.
04 - Add mushrooms and thyme; cook for 5-6 minutes, stirring occasionally, until mushrooms are browned and their liquid has evaporated.
05 - Pour in chicken stock and scrape up any browned bits from the pan. Reduce heat to medium-low.
06 - Stir in heavy cream and Dijon mustard. Simmer for 3-4 minutes until slightly thickened.
07 - Return chicken to the skillet and spoon sauce over the top. Simmer for 2-3 minutes until chicken is heated through and sauce is creamy. Sprinkle with parsley and serve immediately.