Heat the olive oil in a wide, shallow pan over a moderate heat. Add the onion, celery and garlic, and the chilli flakes (if using). Season well with the big pinch of salt and some black pepper and cook for 10 minutes, until soft and fragrant.
While that’s cooking, prepare the garlic bread. Mash the crushed garlic and the oregano into the butter and season with salt and pepper. Use the mixture to butter the slashed bread. Wrap it in foil and place it on a heat source (in the embers of your campfire or in a dry pan) to allow it to warm through and melt the butter.
Add the fennel seeds, oregano and chopped tomatoes to the onion and celery in the pan and continue to cook for another couple of minutes, until rich and thick. Add the stock or water and bring to the boil. Turn the heat down to a simmer and add the fish, then cover with a lid and simmer until the fish is opaque and just cooked through – 5–7 minutes depending on the size and thickness of the fish.
Add the lemon zest and check the seasoning, adjusting with lemon juice and more salt, pepper and chilli flakes accordingly. To serve, ladle the stew into bowls with plenty of the sauce and serve with the garlic bread alongside.