Heat the oil and butter in a large and deep pan and brown sliced or halved if small chestnut mushroom over medium heat until golden, stir once in a while to avoid burning. This will take approximately 5-7 minutes, do not rush that process. Add chopped onion and garlic and cook over lower heat for 5 minutes longer. Remove to a separate plate and set aside.
To the same pan add bite-sized sliced chicken breasts with a pinch of salt and brown on all sides over high heat for 5-7 minutes. When the chicken is browned pour the white wine all over and scrape the bottom of the pan with a spatula to release the brown bits. Then add the chicken stock, mustard, tomato paste and bay leaf.
Return the onions and mushrooms back to the pan, bring everything to a boil and then lower the heat, add the sour cream and chopped parsley and let it simmer uncovered for 5 minutes or until the sauce is thickened to a desired consistency.
You can use ½ cup of sour cream or creme fraiche and ½ cup of double cream/whipping cream if you find the taste of sour cream too strong.