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.
Soy sauce + mirin + honey/sugar. Add some Sake if you have it. That’s the basic sauce for most Japanese rice meals (oyakodon, gyudon, etc.). If you literally just want one sauce, Knorr liquid seasoning works on almost any savory dish.