How to Make It
In a heavy glass or ceramic bowl, whisk the egg yolks with the water, mustard and a pinch each of salt and pepper. Whisk in the oil in small drops until the sauce starts to thicken, then whisk in the remaining oil in a thin, steady stream; when all of the oil has been added, the sauce should be thick. Whisk in 1 tablespoon of the lemon juice. Fold in the egg, cornichons, capers, parsley, tarragon and garlic. Season with salt and pepper and refrigerate.
Preheat the oven to 400°. In a cake pan, toast the walnuts for about 4 minutes, or until fragrant and deep brown.
Bring a medium saucepan of salted water to a boil. Shred the celery root on the largest holes of a box grater and toss immediately with 1 tablespoon of the lemon juice. Add 3 tablespoons of the lemon juice to the saucepan, then add the celery root and blanch until crisp-tender, about 2 minutes. Drain the celery root and plunge it into a bowl of ice water to stop the cooking. Drain again and pat thoroughly dry.
Grate the pears into a bowl and toss with the remaining 1 tablespoon of lemon juice. Add the celery root, remoulade sauce and walnuts and toss well. Season with salt and pepper. Garnish with the celery leaves and serve.