Preheat oven to 450 degrees. Cover a baking sheet with parchment paper. Toss the cauliflower with oil and curry powder until coated and spread cauliflower in a single layer on the baking sheet. Bake for 25-30 minutes or until browned. For the garnish reserve 1 Tbsp. of roasted cauliflower per serving of soup.
Meanwhile, in a large pot, add oil, onion and garlic. Cook over medium heat until onions are translucent and soft. Add the vegetable broth, remaining roasted cauliflower, coconut milk, red curry paste, salt, lime juice and coriander. Heat over medium heat for 15-20 minutes.
Puree the soup with a hand blender or in small batches using a regular blender. Place the soup back on the stove and bring to a boil.
In a small bowl, combine the cornstarch and cold water. Stir to combine. Add to the soup and stir. Cook for 5 minutes over high heat.
Portion: 1 cup of soup, 1 Tbsp of remaining curry roasted cauliflower, 1 tsp scallions