Hazelnut oil has a subtle, nutty flavor that is perfect with winter vegetables. You can also use walnut oil or extra-virgin oilve oil.
Tear frisee into medium pieces. Separate endive leaves. Quarter fennel bulb and slice as thinly as possible, using a mandoline or vegetable slicer. Slice one half of leek thinly crosswise, reserving other half for another use. Peel celery and slice paper thin on the diagonal. Refrigerate vegetables in covered bowls until ready to use.
Toast hazelnuts at 350 degrees until skins blister and they give off a nutty aroma. Wrap in a clean kitchen towel and, when cool enough to handle, rub off skins. Set aside to cool completely.
Whisk together vinegar, mustard, and salt and pepper to taste. Slowly add the oil.
Toss each vegetable separately with a little dressing, and arrange on cold plates. Crush hazelnuts lightly with the back of a knife and sprinkle over each salad. Serve immediately.