Peel the celeriac, then cut it into matchsticks; you can use a food processor or julienne cutter, or cut it into 1.5mm-thick slices, stack these on top of each other and cut into batons of a similar thickness. As you work, periodically squeeze the lemon juice on top of the cut celeriac, to stop it discolouring.
You can now use the celeriac as is, but I like to blanch it, too. Bring a large pan of salted water to a boil, dunk in the celeriac for a couple of seconds, then immediately rinse under cold water, drain well and pat dry.
To make the dressing, put the mustard in a small heatproof bowl, beat in two tablespoons of boiling water, then whisk in the oil and vinegar. Season to taste, stirring in more mustard if you’d prefer a punchier dressing.
Toss the dressing through the celeriac, to coat. You can now serve the remoulade immediately, but I think it’s best left for a while to let the flavours mingle. Top with chopped herbs before serving.
