In a large saucepan, combine the potatoes, 2 cups milk and 1 teaspoon salt. Cover and bring to a boil over medium-high heat. Lower the heat to medium-low, add the cauliflower and scallion whites, cover and simmer until tender, 10 to 15 minutes. Add the remaining 1 cup milk, then puree the soup. Season with salt and pepper.
Microwave the bacon at high power until crisp, 3 to 4 minutes; crumble. Divide the soup among 4 bowls and top with the shredded cheese, sour cream, crumbled bacon and scallion greens.