How to Make It
Heat the oven to 450°. In a large roasting pan, toss the beets with 1 tablespoon of the oil, 1/4 teaspoon of the salt, 1/4 teaspoon of the dill, and 1/8 teaspoon pepper. Cook in the upper third of the oven, stirring once, for 20 minutes.
Remove the pan from the oven and push the beets to one side. Add the potatoes to the pan, next to the beets, and toss them with the remaining 1 tablespoon oil and 1/4 teaspoon of the salt. Return the pan to the oven and cook for 10 minutes. Stir the potatoes and beets, keeping them separate; return the pan to the oven.
Meanwhile, oil a baking sheet. Put the salmon on the baking sheet and sprinkle with 1/4 teaspoon of the salt, the remaining 1/4 teaspoon dill, and 1/8 teaspoon pepper. Put the pan in the oven with the vegetables (after you stir them at the end of Step 2). Cook until just done, about 10 minutes for 1-inch fillets.
Meanwhile, in a small saucepan, bring the cream just to a simmer. Remove from the heat and whisk in the horseradish, the remaining 1/4 teaspoon salt, and a pinch of pepper.
Stir the beets and the potatoes together. Serve the vegetables topped with the salmon and the horseradish sauce.