How to Make It
Combine cream, buttermilk, and vinegar in a small bowl. Cover with plastic wrap, and let stand at room temperature for 8 hours. Stir in dill, 1/8 teaspoon salt, and 1/8 teaspoon pepper. Cover and refrigerate overnight.
Meanwhile, combine remaining 1 teaspoon salt, remaining 1 teaspoon pepper, horseradish, and next 4 ingredients (through oil) in a small bowl. Spread horseradish mixture evenly over salmon. Cover and refrigerate overnight.
Preheat oven to 450°.
Place fish, skin side down, on a parchment paper-lined baking sheet. Bake at 450° for 13 minutes. Remove from oven.
Preheat broiler to high.
Broil fish 5 minutes or until desired degree of doneness. Cut fish into 8 equal portions; top fish with dill sauce.