Chop the fish into 5cm chuncks or, if small, leave whole. Put in a large, flameproof casserole. Add the peppercorns, herbs, wine and stock. Bring to the boil, then reduce the heat to a gentle simmer and cover. Cook for 5-6 minutes.
Using a slotten spoon, remove the fish to a plate and keep hot. Measure the broth - there should be about 550ml or so - if not add extra water. Poor into a jug or bowl.
Add the oil, onion, garlic and chillies to the hot pan, fry for 2 minutes, then add the tomatoes. Return the fish and broth to the pan. Bring back to simmering and serve ladled over chuncks of toasted bread as soup, then ladle out the fish as a stew. Alternatively, serve both at the same time.