To make this in a large pot or Dutch oven, sauté over medium-high heat in steps 1 and 2. For step 3, bring to a boil, then cook, covered, over low heat until the potatoes and cauliflower are tender, 25 to 30 minutes. Transfer 1 cup of the liquid and the whole cauliflower to the blender, and blend until smooth. Stir the puree into the soup, add the cheddar, and serve topped with the scallions.
