Heat 1 tsp olive oil and half of the 25g butter in a large lidded casserole.
Season 4 chicken legs, then fry for about 5 mins on each side until golden brown. Remove and set aside.
Melt the remaining butter in the pan. Add 1 chopped onion, then fry for about 5 mins until soft.
Add 2 crushed garlic cloves, cook for about 1 min, add 200g small button or chestnut mushrooms, cook for 2 mins, then add 225ml red wine.
Stir in 2 tbsp tomato purée, let the liquid bubble and reduce for about 5 mins, then stir in 2 thyme sprigs and pour over 500ml chicken stock.
Slip the chicken legs back into the pan, then cover and simmer on a low heat for about 1 hr until the chicken is very tender.
Remove the chicken legs from the pan and keep warm. Rapidly boil down the sauce for 10 mins or so until it is syrupy and the flavour has concentrated.
Put the chicken legs back into the sauce and serve.