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.
Mayo and dill pickle relish (or chopped dill pickles and pickled jalapenos) is the best tartar sauce IMO.
Chipotle salsa - dump a can of chipotles en adobo into the food processor, press start, turn it off when they are smooth.
Black bean sauce. Dump a can of black beans in a pot with some of the chipotle salsa, heat them together, hit with immersion blender and then whisk in some sour cream.
If you have an abundance of basil (if you have a plant, you will have an abundance) pesto is easy.
Slightly more work but so easy and so good, sheet pan salsa, put tomatillos and fresh hot peppers on a sheet pan and broil them until soft and a little blackened, slide the whole mess into the food processor, season with salt, a little olive oil and vinegar and either cumin seeds or cilantro (you don’t need both), pulse until chopped up and combined.
I find vegan gravy so good for the amount of work too. Make a dark roux of oil and flour, mix soy sauce into water until it looks like diet coke for the broth, add it slowly into the hot roux while whisking, season with pepper, salt if needed, rosemary is good as are mushrooms, miso, but it’s plenty good on its own.