1Place potatoes in medium saucepan and cover with cold water and 1 teaspoon salt; bring to a boil, reduce heat, and simmer until tender, about 15 to 18 minutes.
Drain potatoes and run under cold water to cool off; cut into quarters.
2Meanwhile, in a large skillet, heat 1 tablespoon oil over medium high heat; season salmon with 1/2 teaspoon salt and 1/4 teaspoon pepper. Cook salmon until opaque, about 4 to 5 minutes per side.
3In a large bowl, combine horseradish, vinegar, green onions, 3 tablespoons oil, 1/2 teaspoon salt and 1/4 teaspoons pepper. Add potatoes and watercress, tossing to combine.