Preheat the oven to 425 degrees. Place salmon on a rimmed baking sheet covered with foil or parchment paper. Brush salmon with 1 tablespoon of the olive oil and season with salt and pepper. Roast until salmon is just cooked through, about 10-13 minutes.
Meanwhile, combine radishes, cucumber and scallions together in a medium bowl. Stir in horseradish and remaining 2 teaspoons of olive oil. Season to taste with salt and pepper.
Remove salmon from the baking sheet using a thin spatula, leaving the skin behind. Serve immediately with Cucumber-Radish Relish.
Adapted from Martha Stewart Living magazine, April 2012