Remove the heads and shells from the prawns, but leave the last tail segment of the shell in place. Simmer the heads and shells in the stock or water for 20 minutes, then strain and discard them. Set the stock aside and reserve the prawn meat to add to the stew later.
Bring the vinegar and the sugar to the boil in a small pan and reduce to a couple of teaspoons.
Heat the olive oil and butter in a saucepan, add the garlic, onion, celery and green pepper and sweat for 6-8 minutes. Add the white wine, salt, pepper, chilli flakes, tomatoes, then the fish stock and vinegar reduction. Simmer for 30 minutes, add the oregano for the last 5. You can make this base in advance if you like.
To finish, bring the stew base to the boil. Add the reserved prawns, the monk fish and the mussels, put a lid on the pan and cook for 5 minutes.
Toast the sourdough, then singe the slices slightly on a gas flame. Rub the toast with the garlic and sprinkle with olive oil.
To serve, place a slice of toast in each bowl, ladle in some stew and sprinkle with parsley.