Spread the thinly-sliced onion onto the bottom of a medium-sized saucepan. Separate the smoked haddock from the bones, flaking or cutting it into small pieces; sprinkle the pieces over the onion slices, covering them. Discard the bones.
Carefully stir the pepper and mustard powder into the milk; pour over the flaked haddock and onions. Place the saucepan over medium heat and bring slowly to the boil. Reduce heat to low and simmer, covered, for six minutes. Uncover, and simmer for another five minutes.
Transfer the fish to a warm serving dish with a lid or cover.
Simmer the remaining sauce another five minutes. Gently whisk the butter and finely-chopped syboe (green onion) into the sauce; gradually whisk in the flour. Bring the sauce back to a gentle boil, stirring continually until the sauce thickens; remove from the heat.
Ladle the sauce over the fish. Garnish with the parsley and serve.
