Pat salmon dry and coat with salt and pepper. In a heavy skillet heat 2 tablespoons of oil over moderately high heat until hot but not smoking and sear salmon, skin side down, 5 minutes. Reduce heat to moderately low. Turn salmon and cook 4 minutes more, or until it just flakes.
While salmon is cooking, in a small bowl whisk together vinegar, mustard, horseradish, remaining 4 tablespoons oil, and salt and pepper to taste until emulsified. Serve vinaigrette over salmon.
Adapted from a recipe by Gourmet Magazine