Cook the potato in salted water until fork-tender, 8 minutes. Drain.
Meanwhile, stir together the mayonnaise, horseradish, and 1 tablespoon of the lemon juice in a small bowl.
Heat 2 tablespoons of the oil in a large skillet over medium-high. Add the potato, ¼ teaspoon salt, and ¼ teaspoon pepper; cook, turning occasionally, until brown and crisp, about 10 minutes. Add the onion whites and asparagus and cook, stirring, until the vegetables are tender, about 10 minutes. Transfer to a plate.
Heat the remaining oil in the skillet over medium-high. Sprinkle the fish with the remaining salt and pepper. Cook, in batches, until opaque throughout, 2 to 3 minutes per side. Serve, sprinkled with the remaining lemon juice, the dill, and onion greens, with the asparagus hash and horseradish sauce on the side.
Where did the directions go? :(