Heat oil and butter in a large pan, then brown sliced chestnut mushrooms over medium heat for 5-7 minutes until golden. Add chopped onion and minced garlic, cooking for another 5 minutes before removing to a separate plate.

In the same pan, brown bite-sized chicken breasts with a pinch of salt over high heat for 5-7 minutes. Deglaze the pan with white wine, scraping up any brown bits. Stir in chicken stock, mustard, tomato paste, and bay leaf.

Return the cooked mushrooms and onions to the pan. Bring the mixture to a boil, then reduce heat and stir in sour cream and chopped parsley. Simmer uncovered for 5 minutes, or until the sauce thickens to your desired consistency.