1Wash 3 cups of fresh (or 1 cup dried) elderberries (stems removed) and place in a large nonaluminum soup pot. Add 6 cups of water and bring to a boil. Reduce heat to low and simmer, covered, for 45 minutes or until the elderberries are soft. Blend and then strain through a sieve. Return the strained liquid to the cleaned pot over medium heat, and add 3/4 cup of sugar.
2In a separate bowl, make a slurry by combining 1 tablespoon of cornstarch with 2 tablespoons of water. Whisk the slurry into the pot and bring back to a boil, whisking often.
3Add 2 cups peeled and diced tart apples and 1 tablespoon grated lemon zest, and simmer for 5 minutes or until the soup is thickened, smooth, and clear.
4Cool the soup and refrigerate to chill. Serve cold topped with crumbled gingersnap cookies and dollops of sour cream.