Chop the canned tomatoes by sticking a knife in the jar and stirring, then spoon a layer of chopped tomato into the casserole dish.
Cut the chicken into 3 pieces, jointed at the leg and the thigh cut into two pieces
Heat the oil and brown the chicken pieces.
As the chicken parts are browned, layer them in the casserole, on top of the tomatoes.
Spoon another layer of tomatoes on top of the chicken and put some of the sliced garlic cloves and half of the oregano onto that as well.
Fry the rest of the chicken and layer it on top of the tomatoes.
At this point, the chicken fat will have rendered and there will be too much to use, so drain about half of the pan drippings and use the remainder to fry the onions until they begin to brown, and then add the peppers and cook quickly until heated through.
Put this on top of the second chicken layer and then add the sliced mushrooms on top of this.
Cover and bake at 400 for 25 minutes.
Remove from oven and uncover.
Serve either on pasta or with buttered crusty bread and salad.