Peel the celery root and cut it into pieces. Peel and chop the onion. Peel the apples, remove the cores, and cut them into pieces.

Sauté the onion with 1 tablespoon of curry powder in the olive oil until the onion is translucent. Add the apples and celery root. Cook for a few minutes, then add the vegetable broth and thyme. Bring to a simmer and cook for 30 minutes.

Check for doneness with the tip of a knife—the celery root should be tender. Remove the thyme, add the cream, season with salt and pepper, and blend until smooth.

Serve the velouté drizzled with hazelnut oil and sprinkled with chopped roasted hazelnuts, if desired.