1) Heat half of the butter and the oil in a heavy bottomed pan and brown the chicken pieces on all sides. You may need to do this in batches. Remove and set aside. 2) Add the remaining butter to the casserole and sauté the onions for 3-4 minutes. 3) Add garlic, and mushrooms and cook for 3 mins, then stir in the tomato puree. 4) Add the wine and bring to the boil, allow to reduce for about 5 minutes. 5) Now add the thyme, bay leaf and the stock. 6) Add the reserved chicken to the pot bring to a simmer, cover and cook for 1 hour or until the chicken is cooked. 7) Remove the chicken from the pan and cover with foil to keep warm. 8) Bring the sauce to the boil and reduce for approximately 10 minutes until it is nice and thick. 9) Serve the chicken with the sauce poured over.