Remove the skin from the celeriac, halve the bulb, and slice into very thin julienne slices. Julienne the apple, and toss both in a bowl. In a smaller bowl, whisk the yolk with the vinegar and mustard. Add the grapeseed oil, drop by drop to make a thick mayonnaise. Season with salt, pepper, and lemon juice, to taste. Stir in the fennel seed, and toss with the celeriac to coat. Cover and refrigerate until serving. Cook's Note: Several hours is fine: some would even argue it's de rigueur.