Heat the oil in large pot over medium heat. Add the onions and cook until tender, about 5 minutes. Add the carrots, cauliflower, red pepper flakes, lemon zest, cumin, stock, and 2 cups water and bring to a boil. Cover and simmer over low heat until the carrots are very tender, about 30 to 40 minutes. Let the soup cool slightly, and then blend until smooth, working in batches. Pour the soup into a large bowl and whisk in 1/2 cup of the yogurt. Taste and season with salt and pepper. Chill the soup in the refrigerator for at least 2 hours up to 24 hours. Serve the soup in individual bowls with dollops of remaining yogurt, and some chopped green onion.
When blending hot liquids: Remove liquid from the heat and allow to cool for at least 5 minutes. Transfer liquid to a blender or food processor and fill it no more than halfway. If using a blender, release one corner of the lid. This prevents the vacuum effect that creates heat explosions. Place a towel over the top of the machine, pulse a few times then process on high speed until smooth.