Place the cauliflower florets into a large bowl of lightly salted water; allow to stand for 20 minutes. Drain well, and arrange on a sheet of heavy aluminum foil on a baking sheet. Spray the olive oil cooking spray evenly on the cauliflower.
Preheat the oven's broiler and set the oven rack about 6 inches from the heat source.
Broil the cauliflower until browned, 20 to 30 minutes.
Meanwhile, heat olive oil in a large soup pot, and cook the onion until translucent, about 5 minutes; stir in the garlic and roasted cauliflower. Pour in the water, season with salt and black pepper, and simmer until all the vegetables are tender, about 30 minutes. Blend the soup in the pot with an immersion hand blender until creamy and smooth.
