Turn the oven on to 400 degrees Fahrenheit (200 degrees Celsius). Put parchment paper on a baking pan.

In a large bowl, combine the celeriac, carrots, onion, and garlic. Season with salt, pepper, and Herbes de Provence. Pour some olive oil over it and mix it all together. Arrange the vegetables on the baking sheet that has been prepared.

Vegetables should be roasted in a warm oven for approximately 40 minutes, stirring every 19 to 15 minutes.

Put the roasted veggies in a powerful blender. Blend on high speed for approximately one minute after adding four cups of chicken broth.

Over medium heat, pour the mixture into a large saucepan. Stir in the remaining chicken broth and cook until thoroughly heated. Add heavy cream and stir. Spoon soup into dishes and sprinkle with green onions and a little truffle oil for garnish.