Step 1 Toast hazelnuts on baking sheet at 350 degrees until skins start to loosen, about 5 minutes. Place nuts in towel and rub them so that skins loosen and can be removed. Chop nuts and set aside.
Step 2 Combine egg, mustard, salt and lemon juice in blender until combined. Add olive and canola oils slowly in thin steam until mayonnaise is thickened and fluffy. Stir in sour cream, tarragon and pepper to taste.
Step 3Toss together chicken, onion and celery. Stir in mayonnaise until blended. Stir in hazelnuts. Add salt to taste. Chill 1 hour.