Preheat oven to 375°F with a rack in the upper third position. Season the salmon with the salt and white pepper. Place on a large piece of foil and close to make a loose-fitting, tightly sealed package.
Put the salmon on a baking sheet and cook for 20 to 25 minutes, until the fish is still slightly rare in the center (it should be only slightly resistant to the touch). Carefully open the foil and let the fish rest for a few minutes. Or let cook completely and chill before serving.
Meanwhile, combine the mayonnaise, lemon zest and juice, scallion, caper, dill, and parsley. Season to taste with salt.
Cut the fish into 4 pieces and serve with a dollop sauce on the side.