Put the porcini mushrooms in a bowl, add 1 cup (8 fl oz/250 ml) warm water and soak for 20-30 minutes. Squeeze the porcini over the bowl. Strain the liquid and reserve. Finely chop the porcini. C
Heat the oil and butter in a flameproof casserole. Saute the chicken over a medium heat for 5 minutes. Drain on kitchen paper.
Add the onion and chopped porcini to the pan. Cook gently, stirring frequently, for about 3 minutes until the onion has softened but not browned. Stir in the chopped tomatoes, wine and reserved mushroom soaking liquid, then add the crushed garlic and chopped rosemary, with salt and ground black pepper to taste. Bring to the boil over a medium heat, stirring all the time with a wooden spoon.
Return the chicken to the pan and spoon the sauce over the top. Cover the pan and simmer gently for 30 minutes.
Add the fresh mushrooms and stir well to mix into the sauce. Continue simmering gently for about 10 minutes, until the chicken is tender. Taste for seasoning. Serve hot, with creamed potato or polenta, if you like. Garnish with rosemary.