There is some debate over whether this dish is a Scottish or New England original, but apparently no argument about how delicious it is. Here's a recipe from Edible Cape Cod.
Ingredients:
3 T butter, softened
2 t AP flour
sunflower oil
1 pound smoked haddock, cut into 2 or 3 large pieces
1 large yellow onion, thinly sliced
1-2/3 cup whole milk
Freshly ground black pepper to taste
1 green onion or scallion, finely chopped
Handful of flat parsley, finely chopped
Optional: 1 cup grated cheddar cheese and hardboiled eggs, to serve
Mix a beurre manié, by combining the softened butter and flour to make a smooth paste and set aside.
Heat oil in a large sauté pan over medium heat.
Place onion in a pan and cook for a minute or two.
Place fish over onions in pan. Add the milk and a few grindings of pepper.
Bring milk to a gentle simmer. Reduce heat to medium low, cover pan and cook for about 5 minutes.
Remove fish, leaving milk mixture in pan, and let cool for a minute or so.
Add green onion to milk in the pan and continue to gently simmer for another 5 minutes.
Meanwhile, cut or break fish into strips about 1-1/2 inches wide and set in a warmed serving dish.
Remove pan with milk from heat—either turn off heat or move pan off burner. Whisk beurre manié into milk and return to medium heat, stirring constantly. Bring milk back to a simmer—it should thicken slightly.
Add cheddar cheese, if using, and heat for a minute or two to incorporate into sauce.
Check sauce consistency and if too thick add milk and heat for a minute.
Stir in chopped parsley and pour sauce over reserved fish.
