To prepare the salmon, season it on both sides with salt. Spread the pepper on a work survace and press the flesh side of the salmon into it until all the pepper adheres. Cut the fillet into 4 equal pieces and place them on a platter; cover with plastic wrap and refrigerate.

To make the potatoes, preheat an oven to 400F. Prick the potatoes in several places with the tines of a fork, place them on a baking sheet in the oven, and bake until tender 40 to 50 minutes.

While the potatoes are baking, make the horseradish cream: In a small saucepan over medium heat, combine the cream, butter, salt, and white pepper. Bring almost to a boil and remove from heat. Stir and let cool for 10 minutes. Add the grated horseradish and transfer to a blender. Process until smooth, about 30 seconds. Pour the cream into the original saucepan and cool thoroughly. Strain through a fine-mesh sieve into another saucepan.

When the potatoes are done, reheat the horseradish cream over medium-low heat. Remove the potatoes from the oven, but leave the oven on. Cut the potatoes in half and scoop out the pulp into a bowl. Add the horseradish cream and, using an electric mixer, beat until completely smooth. Keep warm.

In a frying pan over high heat, warm the vegetable oil. Place the salmon pieces in the pan, flesh side down. Sauté until golden brown and almost crisp, 3 to 4 minutes. Carefully turn over the fish and place the pan in the oven. Roast for 4 to 5 minutes, transfer the fish to a warmed platter and drain the oil from the pan. Add the butter and shallot, place over medium heat and saute for 1 minute. Return the fish to the pan, add the chopped parsley and lemon juice and heat through.

To serve, divide the potatoes among warmed individual plates, top each with a peice of fish, drizzle with some of the pan sauce and garnish with the parsley sprigs.