Pickled radishes and homemade croutons add a satisfying crunch to the velvety soup made primarily of sunchokes.
Pickled Radishes: Place radishes in large heat-proof bowl. Bring vinegar, sugar, and 2 cups water to a boil in saucepan. Pour vinegar mixture over radishes, and cool to room temperature. Slice radishes into thin rounds.
Croutons: Heat oil in large skillet over medium heat. Add bread; cook, stirring occasionally, 7 to 10 minutes, or until browned and crispy. Drain on paper-towel-lined plate.
Sunchoke Soup: Heat oil in large saucepan over medium heat. Add onion, and sauté 3 to 5 minutes, or until soft and translucent. Add sunchokes, sugar, and 5 cups water. Season with salt, if desired. Simmer 30 to 35 minutes, or until sunchokes are very soft. (At this point the liquid should be reduced by about half.)
Remove from heat, and purée in food processor or using handheld blender until smooth. Strain through fine-mesh strainer.
Pour warm soup into bowls, and garnish with Pickled Radishes and Croutons.