In large pot, bring 7 cups stock, tomatoes and their juices, and beets to boil. Cook over medium-high until tender, 35 minutes; transfer to blender. Puree, adding more stock if needed to thin; season. Refrigerate overnight. In bowl, whisk horseradish sauce with 1 1/2 tbsp. water. Divide soup among bowls, drizzle with sauce and top with dill.