Place the fish fillets in a frying pan, cover with cold water, bring to a simmer, cover and cook for about 6 minutes, or until the fish flakes easily with a fork. Cool in the stock, remove and flake. Set aside. Discard the stock, rinse out and dry the pan.
Heat the butter in the pan and sauté the onion until tender but not browned. Add the tomatoes, salt and pepper and cayenne pepper an