I think I was wrong on ingredients- oops. Don’t know if it’s tartar sauce either, but I can to this day recite the “jingle” or whatever for the Big Mac. Used to be able to do it backwards 
I think it is more like Russian/thousand island than tartar sauce. Tartar sauce on Filet-O-Fish.
Trader Joe’s sells a “special sauce” that is quite like the Big Mac sauce - maybe with a little more kick?
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