There are a number of thickeners easily available to the home cook, however many are not a wise choice for thickening Italian dressing. Roux and other flour based thickeners result in a dressing that seems a lot more like sour, oily gravy than dressing. Cornstarch, arrowroot and similar starch-based thickeners, though clear and with little flavor, must be heated to thicken liquids. Xanthan gum, however, is easy to use, does not require heating to release its thickening properties and has little to no flavor.
Combine 1/8 of a teaspoon of xanthan gum per cup of prepared Italian dressing in a large mixing bowl.
Whisk the dressing thoroughly and vigorously until thickened -- this happens quickly.
Check the consistency once the dressing doesn't thicken any more through whisking.
Add 1/8 teaspoons more xanthan gum per cup of liquid if the dressing is still not thick enough, and repeat whisking until thickened to your desired consistency.