Roasting a side of salmon is a quick, simple way to feed a group. Use tweezers or small pliers to remove any remaining pin bones your fishmonger may have overlooked.
Preheat oven to 425 degrees. Place salmon on a parchment-lined baking sheet, and drizzle with 2 teaspoons oil. Season with salt and pepper. Roast salmon until just cooked through (it will be opaque around edges), 8 to 10 minutes.
Meanwhile, stir together radish, cucumber, horseradish, scallions, and remaining teaspoon oil until combined. Season with salt and pepper. Serve with salmon.