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 (several hours is fine: some would even argue it’s de rigueur).