Season the flour with salt and pepper. Toss the chicken pieces in the flour.
Melt the butter in a frying pan on the hob and fry the chicken pieces and onion for 5 minutes or until beginning to brown. Pour in the stock, stir through and bring to the boil. Transfer to a casserole dish and add the rosemary, mushrooms and a little salt and pepper. Cover with foil and place on the lower rack of the halogen oven.
Set the temperature to 200°C (392°F) and cook for 40 minutes, removing the foil for the last 5 minutes of cooking time. Remove from the oven and stir in the crème fraîche. Check the seasoning and serve with celeriac and potato mash and seasonal vegetables.