Working quickly, peel the celery root and cut into chunks. Grate finely in a food processor fitted with the shredding blade and immediately toss with the salt and lemon juice. Set aside.

Place the mustard in a mixing bowl and, while whisking, slowly drizzle in the hot water. When the water is completely incorporated, drizzle in the oil, little by little, until completely incorporated. Add the vinegar, little by little, to form a smooth emulsion and season with white pepper. Stir in the herbs, gherkins, and capers. Add the dressing to the celeriac, to taste. Serve immediately or allow to sit, refrigerated, in a nonreactive airtight container, for 2 hours and up to 2 days.