Soak the porcinis in 1/2 cup boiling water for 20 minutes. Remove the porcinis (reserving soaking liquid) and dice. Meanwhile, preheat the oven to 180 degrees C.
Heat the oil in a casserole dish. Coat the chicken with the flour and brown 4 minutes on either side. Remove and set aside.
Reduce heat, add the onion and garlic and cook until onion is soft, about five minutes. Add mushrooms and cook 5 more minutes. Add porcinis and soaking liquid and simmer 2 minutes. Add tomatoes and salt and pepper to taste.
Return the chicken to the casserole dish, cover and bake in the oven 35 minutes. Meanwhile, cook spaghetti until al dente. Serve topped with the chicken.