- 1 russet potato, cut into ¾-in. pieces
- ½ teaspoon kosher salt, divided
- ½ cup mayonnaise
- 3 tablespoons horseradish
- 2 tablespoons fresh lemon juice, divided
- ¼ cup extra-virgin olive oil, divided
- ½ teaspoon black pepper, divided
- 3 spring onion bulbs, whites quartered and greens thinly sliced
- 1 bunch asparagus, cut into 2-in. pieces
- 4 (6-ounce) pieces boneless, skinless cod fillets
- 2 tablespoons roughly chopped dill
- 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.