Cut cauliflower into medium-sized pieces and place in a baking dish. Drizzle with a mild-flavored oil and sprinkle with a little salt and pepper. Bake at 400 degrees for ~30 minutes until it's roasted.
While the cauliflower is roasting, chop the garlic and onion. Once the cauliflower is done, you can begin making the soup. Add a bit of mild-flavored oil to a large pot over medium heat, then saute the garlic and onion until the onion is translucent and the garlic is golden brown.
Add the curry paste to the cooked onion and garlic, and mix to incorporate. Once it has had a minute or two to fry, add the roasted cauliflower and mix together with the other ingredients.
Next add the chicken broth, water, and coconut milk. Mix together, then let everything simmer until the cauliflower is completely soft (~15 minutes).
Season the soup with with fish sauce and sugar as needed. The amount you will need to use will depend on which curry paste you used, so taste and adjust accordingly.
Take the soup off the heat, and blend until it is smooth and creamy. Pour into individual serving bowls and serve with a sprinkling of dried chili flakes and green onion for garnish.