Step 1

Preheat the oven to 400 degrees F (200 degrees C). Line a baking sheet with parchment paper.

Step 2

Combine celeriac, carrots, onion, and garlic in a large bowl. Sprinkle with Herbes de Provence, salt, and pepper. Drizzle with olive oil and stir to combine evenly. Spread out vegetables on the prepared baking sheet.

Step 3

Roast vegetables in the preheated oven until cooked through, stirring every 19 to 15 minutes, about 40 minutes total.

Step 4

Place roasted vegetables into a high-powered blender. Add 4 cups chicken broth and blend on high speed until smooth, about 1 minute.

Step 5

Pour mixture into a large saucepan over medium heat. Add remaining chicken broth and heat, stirring, until warmed through. Stir in heavy cream. Ladle soup into bowls and garnish with a light drizzle of truffle oil and a sprinkling of green onions.