Melt the butter in a large pan and gently fry the onion for about 5 minutes until transparent. Add all the sweetcorn, milk and potatoes and season with black pepper. Bring to simmering point and continue to cook over a low heat, stirring occasionally, for about 20 minutes or until the potato is tender.

Cut the fish into large pieces, add to the chowder and simmer for another 3 minutes. To serve, ladle the chowder into warm bowls and garnish with the tomato and thyme.