Heat the olive oil and butter in a large casserole dish over a moderate heat until melted. Season then sear the chicken legs until golden in colour all over. Remove using tongs and drain on kitchen paper.
Lower the heat a little and add the onion, garlic and sliced mushrooms, stirring occasionally until the onion has softened but not coloured. Add the tomatoes, wine, bay leaf and seasoning. Bring to the boil, then reduce to a simmer. Add the chicken legs back to the casserole dish, cover and simmer for 25-30 minutes.
Remove the cover and simmer for a further 10 minutes until the chicken is cooked. Cut into a thick part of the leg to check that the chicken is cooked through. Check the seasoning then allow to stand for 5 minutes. Garnish with sprigs of rosemary before serving from the casserole dish.
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