Heat the sunflower oil and butter in a large, heavy-based casserole dish over a moderate heat. Season the chicken pieces then sear until golden brown in colour all over. Remove from the dish, then add the onion and garlic and sweat for 3-4 minutes, until they start to soften a little.
Deglaze the dish with the white wine, scraping the bottom to loosen any debris. Add the stock and mushrooms to the dish and bring to the boil. Return the chicken to the dish along with the bay leaf and rosemary. Cover with a lid and reduce to a simmer for 25-30 minutes until the chicken is cooked through with no pink showing.
Once cooked, stir in the crème fraîche and adjust the seasoning if necessary. Discard the bay leaf and rosemary before serving straight from the dish.
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