Cut cauliflower into florets. Toss with a tablespoon or two of oil, plus a little salt and pepper, and spread out on a baking sheet. Roast at 375 degrees for about 10 to 12 minutes, or until the cauliflower starts to turn golden. Remove from oven and cool slightly. Toast the cumin seed and crust it with the coriander using a mortar and pestle. Swirl in a half teaspoon each salt and black pepper.
Heat a large soup pot over medium heat and add 2 tablespoons olive oil. Add onion and red pepper and cook for about five minutes. Add garlic and red pepper, stir to combine, and cook a couple minutes more. Add the spice-seasoning mixture, broth, roasted cauliflower and diced potatoes to the pot. Bring to a boil, stir and reduce heat to a simmer. Continue cooking for about 15 to 20 minutes, until vegetables are tender.
Turn off heat and mash vegetables slightly with a heavy spoon or potato masher. Using a slotted spoon, remove/reserve about two cups vegetables from the soup pot. Puree the soup using an immersion blender. Return the vegetables to the soup pot. Turn heat to low-medium and slowly add milk to pot until soup reaches the consistency (and color) you want. Heat slowly; do not boil. Taste for seasonings and add salt and pepper as needed. Ladle soup into heated bowls. Garnish with a sprinkle of Parmesan OR swirl in a little sour cream, if desired. Makes 12 or more servings, depending on size.