Preheat broiler. Line a rimmed baking sheet with foil.
Microwave the spinach according to package directions until wilted.
Arrange the salmon on prepared pan, skin-sides down. Season with salt and pepper. Broil salmon 5–6 min., until cooked through.
To a small bowl, add the sour cream, 1 tbsp water, horseradish, and dill.
Whisk to combine. Season with salt and pepper to taste.
To serve, divide spinach among 4 plates.
Top with salmon and drizzle with sauce.