I saw Bayliss do this on one if his PBS shows (IIRC something about a street vendor food cart at the beach), and I thought I also read it on one of these boards, but could not find it again.
You just take a couple of big spoons of mayo and add water until it becomes like heavy cream. Boy, was it good not to mention simple. It is one of the best 'secret' recipes I have stumbled onto in quite a while.
We had some baked salmon. Kraft mayo + water + a few fresh oregano leaves. Also, we tried a mexican hot sauce and some fresh ground pepper instead of oregano. I now have a 'standard' seafood/fish sauce for this summer.
I can see also sorts of possibilites: add whatever flavor, herb, spice is at hand. Funny thing, it does not taste like mayo anymore. It is rich and creamy but not overly so, with just a faint hint of acid.