1.Bring a large saucepan of salted water to the boil.
2.Cut celeriac into thin matchsticks, using a V-slicer, mandolin or sharp knife. Drop the celeriac into boiling water; cook for 2 minutes. Drain immediately. Rinse the drained celeriac under cold water to prevent it cooking any further; drain well.
3.Whisk the mustard in a medium bowl; gradually whisk in the olive oil and the grape seed oil, in a thin stream, whisking constantly. Whisk in the juice; season to taste
4.Add drained celeriac, parsley and tarragon to the dressing in the bowl; stir until combined. Season to taste.
5.Serve the remoulade with bread, olives, radishes, cornichons and salami, if desired.
Not suitable to freeze or microwave. Leftover remoulade is delicious served on grilled fish or chicken, or added to a chicken sandwich.Note