Heat oil in a large saucepan over medium heat.

Add chopped onion and sauté until tender.

Add diced potatoes, boiling water, salt, and pepper to the saucepan.

Reduce heat, cover, and simmer for 15 to 20 minutes or until the potatoes are tender.

Add the chunks of fish to the simmering mixture.

Simmer until the fish flakes easily with a fork, about 5 minutes.

Just before serving, add milk and butter to the chowder.

Stir well and heat through.

Adjust seasoning if necessary and serve hot.