Place a large soup pot over medium-high heat. Add olive oil and sauté onions until translucent. Add broth and bring to a boil. Add potatoes and cook for 8 minutes or until tender.
Add corn kernels, red peppers and spices to pot and cook for 2 minutes longer. Remove from heat; pour soup through a strainer, reserving vegetables and broth.
Place half of the vegetables in a bowl, and gently mix in the yogurt. This will help to temper the yogurt and prevent it from separating. Combine the remaining vegetables and broth in a blender, and purée until smooth.
Place the puréed vegetables and broth mixture in the soup pot over medium heat, and then stir the yogurt and vegetable mixture back into the pot. Serve warm and garnish with a dollop of plain yogurt.