Heat the oil and half the butter in heavy bottomed pot. Rub the chicken legs all over with salt and pepper and fry until golden brown on all sides, about 5 minutes. Remove from the pot and set aside.
Add the remaining butter to the pot and melt. Add the onion, and fry at low heat, stirring, until it is soft, about 5 minutes. Add the garlic and fry for about 1 minute. Add the mushrooms, fry for 2 minutes then add the wine and the tomato puree. Bring to the boil and let the liquid reduce for about 5 minutes. Stir in the thyme and add the stock. Return the chicken back to the pot, cover and simmer on a low heat until the meat falls off the bones, about 1 hour.
Remove the chicken from the pot and keep warm. Let the sauce boil until it has thickened, about 10 minutes. Return the chicken to the sauce, season to taste and serve.