Heat the butter and oil in a flame-proof casserole and cook the chicken over a medium-high heat, turning frequently for 10 minutes, or until golden all over, then transfer to a plate.
Add the onions and garlic and cook over a low heat, stiring occasionally for about 10 minutes.
Add the tomatoes, herbs, sun-dried tomato paste and wine and season to taste.
Bring to the boil, then return the chicken portions to the casserole, pushing them down into the sauce.
Cover and cook in the preheated oven for 50 minutes.
Add the mushrooms and cook for a further 10 minutes, or until the chicken is tender and the juices run clear when a skewer is inserted.