Heat the oil and butter in a large pan over a medium heat. Add the chicken, salt and pepper and cook until lightly browned.
Add the mushrooms, red onions and garlic and cook until softened.
Pour in the stock, tomato purée, mustard and smoked paprika. Bring to a boil, then reduce the heat to low.
Add the soured cream and chopped parsley and cook for 10 minutes, stirring occasionally, or until the sauce has thickened.
Check the seasoning and add salt and pepper to taste.
