coat a large pot with cooking spray and set it over medium heat. When the pot is hot, add the oil, onion, and then the garlic, and cook, stirring occasionally, until the onion is soft but not brown- 5-6 minutes.
Add the broth and potatoes and bring to a boil. Reduce the heat and simmer, partially covered, until the potatoes are tender, 20-25 minutes.
Carefully spoon the mixture into a blender or food processor and add the vegetable purees, buttermilk, and salt; puree until smooth. Ladle into bowls and sprinkle with cheese if you like.