How to Make It
In a large bowl, whisk the olive oil with the lemon juice and salt. Add the celery root and toss to combine. Refrigerate for 1 hour and up to 6 hours, until the celery root has softened slightly.
Meanwhile, in a small bowl, whisk the mustard with the vinegar, then gradually whisk in both oils until incorporated. Whisk in the cream and season with salt and white pepper. Stir in the minced chives.
Add the endives and avocados to the celery root and toss to combine. Add half of the vinaigrette, season with salt and white pepper and toss. Serve, passing additional vinaigrette at the table.