One of those recipes that you can make a day ahead and get out of your way and not be pressed for time the day of your dinner party or social event. I have had this recipe for over 20 years and it is still one of my favorite veggie recipes. It seems to go really well with everything I've paired it with, but is a real hit with Italian mains. I don't know why, but I have never even added the optional bacon and cheese toppings; it just didn't seem to need them.
In a 9 X 13 pan, layer all the vegetables in the order listed above.
Combine all but mayonaise of the remaining ingredients in a sauce pan. Stir CONSTANTLY over medium-low heat until the mixture is thick. Remove from heat. Use mayonaise, a little at a time, to thin the mixture down to about cake batter consistency. Allow to cool and spread completely over layered vegetables in pan.
Top dressing with crumbled bacon and/or shredded cheese, if desired. Cover and refrigerate for 24 hours.