I think that seasoning is SO important when cooking for one–it can take that serving of veggies from sort of humdrum to great so easily. One of the easiest ways to “dress up” leftovers is to use a sauce with them on the second run.
I think that one of the easiest is a vinaigrette. It’s so simple, holds well in the fridge. If you know a basic ratio (usually 3 parts oil to 1 part acid or if you prefer a less tart dressing 4 parts oil to 1 part acid) you can vary it easily.
A bit of mustard serves to aid the emulsification of the oil and acid. The acid can be vinegar, or it can be lemon juice, lime juice, orange or grapefruit juice. You can easily add different herbs, or garlic, onion, shallot, spices, or chives as an aromatic, depending on what you want: with fish, lemon might be a good choice.
Stored in a small jar, covered, it will keep for approximately a week in the refrigerator, so it’s always handy. This same vinaigrette is good to make a non-mayonnaise tuna or chicken salad. Better and less expensive than store-bought salad dressing, and without additives and preservatives.