Caramelize onions until deeply browned, stirring often. Add water, a tablespoon at a time, to prevent burning.
Once onions are ready, add the soy sauce and balsamic vinegar and cook for 30 seconds, deglazing the pan.
Transfer the onions to a medium bowl and scrape the frying pan to get all of the glaze.
Cool the onions, then stir in the mayonnaise and sour cream. Chill the dip, covered, for 30 minutes or more to allow the flavors to blend.
Nutrition facts are sometimes provided below and are calculated using an online calculator. With specific brands of ingredients and additions, omissions, or substitutions the nutrition facts may change. We encourage you to use your own nutrition facts caculator to obtain the most accurate nutrition facts for your meal.