Preheat the oven to 425°F. Toss the cauliflower florets with 2 tablespoons olive oil, then spread them on a baking sheet. Roast for 25 minutes until the edges turn golden.
Heat the remaining tablespoon of olive oil in a large pot over medium heat. Add the diced onion and cook for 5 minutes until softened.
Stir in the minced garlic and cook for 1 minute until fragrant.
Add the roasted cauliflower, vegetable broth, salt, and black pepper to the pot. Bring to a simmer and cook for 10 minutes.
Remove from heat and blend the soup until smooth using an immersion blender or by transferring to a countertop blender in batches.
Return the soup to low heat and stir in the heavy cream. Warm through for 2 minutes without boiling.
Ladle into bowls, drizzle each with truffle oil, and top with chopped chives before serving.