Dust the fish inside and out with flour and season with salt and pepper. Shake off any excess flour. Heat the oil in a large frying pan and fry the fish until golden-brown on each side, about 3-4 minutes. Set to one side.
In the same pan, fry the onion, garlic, parsley, anchovies and soaked porcini for five minutes, then add the tomatoes and season with salt and pepper to taste. Cook for a further 10 minutes.
Return the fish to the pan and heat through in the sauce for a few minutes to finish. Serve with some good bread or a purée of potatoes.