
Fill a large bowl with cold, lightly salted water. Stir in the cauliflower florets and soak for 30 minutes. Drain and pat dry.
Adjust the oven rack to 6" from the top of the oven and preheat the low broiler. Line a baking sheet with heavy-duty aluminum foil. Layer the cauliflower on the baking sheet evenly and spray with the spray olive oil. Broil for about 25 minutes, until lightly browned. But watch it so it doesn't burn.
Meanwhile, heat the oil in a large stockpot over medium heat and saute the onions until they begin to caramelize, about 15 minutes. Stir in the roasted cauliflower and garlic and toss over medium heat for 2 minutes. Add the water and chicken broth.
Simmer, covered, for 40 minutes or until all the veggies are soft enough to be pureed. Using an immersion blender, puree the soup until smooth and silky Season with salt and pepper, to taste.