Begin by soaking the cauliflower florets in a large bowl of lightly salted water for approximately 20 minutes. This step cleans the florets thoroughly and enhances their flavor. Drain well and pat dry to remove any excess moisture.

Preheat your oven’s broiler and position the oven rack about 6 inches from the heat source. Line a baking sheet with heavy-duty aluminum foil for easy cleanup. Arrange the cauliflower florets in a single layer on the prepared sheet and spray evenly with olive oil cooking spray. Broil for 20 to 30 minutes, turning occasionally, until the cauliflower is golden and caramelized.

In a large soup pot, heat the olive oil over medium heat. Add the chopped onion and sauté until soft and translucent, about 5 minutes. Stir in the garlic and cook for an additional minute until fragrant.

Add the roasted cauliflower to the pot and pour in the water (or vegetable broth if using). Season with salt and pepper to taste. Stir well to combine.

Bring the mixture to a gentle boil, then reduce the heat to medium-low and let it simmer for approximately 30 minutes. Ensure the vegetables are fully tender.

Using an immersion blender, purée the soup directly in the pot until smooth and creamy. If an immersion blender is unavailable, carefully transfer the soup in batches to a standard blender. Take caution with hot liquids and do not overfill the blender.

Ladle the soup into bowls and serve warm. For added flavor, garnish with a drizzle of olive oil, fresh herbs, or a sprinkle of red pepper flakes if desired.