Drain the beets, reserving the liquid. Add enough broth to beet liquid to equal 2 cups. Place the beets and 2 tablespoons of the liquid in a food processor. Process the beets for 20 to 30 seconds or until completely pureed.
Whisk together the beet liquid/broth mixture, flour, sugar, onion powder, and lemon juice in medium saucepan. Cook over medium heat until mixture boils; simmer 1 minute, whisking constantly.
Remove from the heat. Whisk in the pureed beets. Whisk in the sour cream and serve slightly warm immediately or cover and refrigerate 30 minutes and serve chilled.
Thin the remaining 2 tablespoons of sour cream with 2 teaspoons milk. Drizzle over individual servings of the soup. Garnish with herb sprigs, if desired.