Preheat oven to 400°F (200°C).

Toss cauliflower florets and chopped onion with olive oil, salt, and pepper on a baking sheet.

Roast for 20-25 minutes, or until cauliflower is tender and slightly browned.

In a large pot, combine roasted cauliflower and onion with chicken broth.

Bring to a simmer and cook for 10 minutes.

Use an immersion blender to blend the soup until smooth and creamy.

Stir in heavy cream and heat through.

Serve hot.