Peel and core 2 apples, then cut into 1-inch pieces. Heat oil in heavy large pot over medium-high heat. Add shallots; saute 3 minutes. Add parsnips; saute 3 minutes. Add apple pieces and coriander; stir 1 minute. Add 5 cups broth; bring to boil. Reduce heat and simmer until vegetables are very tender, about 25 minutes. Cool slightly. Working in batches, puree soup in blender until smooth, thinning with more broth by 1/2 cupfuls as desired. Return soup to pot; bring to simmer. Season to taste with salt and pepper. Meanwhile, cut remaining apple (with peel) into thin slices. Ladle soup into bowls. Drizzle soup with yogurt. Fan several apple slices on top and serve.