In a pinch when out of tartar sauce I’ve mixed green relish with mayo and it’s, passable.
For that matter, mayo with hot sauce and ketchup was my accompaniment to some otherwise bland pizza and tater tots this afternoon.
Microwave Hollandaise sauce. Can go wrong easily, but when I wanted it quick it works fairly well. Just have to tend the microwave every 15-20 seconds to mix it before it coagulates. But it’s done its job more than once for me.
Lotta mayo+ contraptions here so I’ll add mine, and hard agree with the lemon in handmade tartar sauce it makes all the difference.
Mayo and Cajun seasoning, or if going for more Mediterranean, mayo and greek seasoning if you don’t have sub oil, or if you do mayo and sub oil. All great ways to get moisture on a sandwich