Lemon and herbs infuse salmon with loads of flavor as it roasts.
Preheat oven to 450 degrees. Make the salmon: Using a sharp knife, cut slits crosswise along both sides of salmon, spacing each about 2 inches apart. Stuff each slit with 1 lemon slice and 1 sprig of each herb. Rub salmon all over with oil, and season with salt and pepper. (If making a side of salmon, cut slits into flesh, and stuff with some of the lemon slices and herbs. Do not oil or season.)
Season cavity with salt and pepper, and stuff with scallions. Transfer salmon to a parchment-lined baking sheet. (If making a side of salmon, top flesh with scallions, drizzle with oil, and season with salt and pepper. Transfer salmon to baking sheet flesh side up.) Roast for 10 minutes. Reduce oven temperature to 300 degrees. Roast salmon until flesh is slightly firm to the touch and the tip of a paring knife inserted into the center comes out hot, 30 to 35 minutes. (Salmon will be medium-rare after 30 minutes. If making a side of salmon, roast for 6 to 8 minutes at 450 degrees.) Loosen fish from skin using a metal spatula.
Meanwhile, make the horseradish-caper cream sauce: Mix together sour cream, horseradish, lemon juice, chives, and capers. Season with 1/2 teaspoon salt and some pepper. Serve salmon with sauce.