Parboil the potatoes in salted water for 5 minutes. Drain and set aside until they are cool enough to handle. Cut them into thick rounds.

Heat the oil in a large, deep frying-pan. Add the onion and garlic and fry until they are soft. Add the potato rounds and fry gently until they are evenly browned. Pour over 300 ml (10 fl oz) (1 1/4 cups) of the fish stock and bring to the boil. Reduce the heat to low and simmer the mixture for 20 minutes.

Meanwhile, arrange the fish fillets in a shallow saucepan and pour over the remaining stock. Poach the fish over gentle heat for about 15 minutes, or until the flesh flakes easily. Drain and discard the stock.

Mix the vinegar and cayenne together and carefully stir into the potato mixture. Simmer for 1 minute. Remove from the heat and transfer the potato mixture to a shallow, warmed serving dish. Arrange the fish fillets on top and serve at once.