Heat the oil over moderate heat in a wide saucepan. Add the onion, olives and garlic and sauté until the onion is soft. Add the potatoes, tomato paste, wine, tomatoes and stock, and mix well.Bring to the boil, simmer 5 minutes, taste and season with salt and pepper. Add the fish, mix carefully, and simmer 4 minutes until the fish is cooked through. Serve with some crusty bread or toast. Serves 4-6.