Parsnip and Potato Soup with Crisp Diced Bacon

I like to make this soup in the fall and early winter when parsnips are at their peak. They must remain in the ground until after the first frost, which converts their starches to sugar. In fact, parsnips were once (incorrectly) considered poisonous if they were pulled before the ground had frozen. Deeply flavored and aromatic, this soup is perfect cool-weather comfort food.