In a Dutch oven, melt the butter over medium heat. Add the chopped onions and sauté for about 5 minutes until tender and translucent.

Stir in the chopped potatoes, season with salt and pepper, then pour in enough water to cover the vegetables. Add the haddock on top without stirring, cover, and simmer for 20-25 minutes.

In a separate saucepan, heat the milk over medium-low heat, then mix in the remaining butter and evaporated milk until creamy and well-combined.

Pour the heated milk mixture over the chowder in the Dutch oven, stir gently to combine, and heat through for 5 minutes. Adjust seasoning as needed.