This really is . . . fast and simple; in fact, so much so that I've stopped any other form of cooking apart from stirfry and salad and an occasional egg on toast.
One pan, sauté onion, throw in any veg you like (if potato, thinly sliced) stir it about a bit, add a chunk of fish or an egg, if desired; add some water/soy sauce/seasoning/ put lid on and go and do something else for about twenty minutes.
Voila - a single pan to wash up and a plate/fork/spoon.