Heat the butter in a large saucepan or Dutch oven over medium-low heat. When the butter is hot, add the diced onion, 1 1/2 teaspoons of kosher salt, and pepper, and cover the pan. Cook, stirring occasionally, until the onions are transparent, about 6 to 8 minutes. If the onions begin to brown, lower the heat to the lowest setting.

While the onions are cooking, peel and dice the potatoes into 1/2-inch cubes. You should have 4 to 4 1/2 cups.

Add the potatoes to the cooked onions and add enough water to cover them. Increase the heat to medium-high and bring the potatoes to a boil. Cover the pan, lower the heat to medium-low, and continue to cook until the potatoes are fork-tender, about 12 to 15 minutes. Do not drain.

Place the haddock fillets on the potatoes, cover the pan, and continue cooking over medium-low heat until the fish is opaque and flakes easily with a fork, about 10 minutes.

Add the evaporated milk to the pan and stir gently. Cook until the chowder is hot.

Taste the chowder and adjust the seasonings with more salt and pepper, as needed.

Garnish bowls of hot haddock chowder with bacon, parsley, or chives, and serve with oyster crackers or saltines on the side.