Cut the fish into chunks, bones, head and all (remove gills first). Heat the olive oil until smoking, add garlic and onions, toss for a minute or two, add the sliced tomatoes, herbs and fish including shells, cook for 10 minutes, then add enough fish stock or water to cover. Bring to a fast boil and cook for a further 10 minutes. Add more liquid if it reduces too much.
Soak the saffron