[Oubliette]

Funnily enough, I actually first played keyboard rhythm games one-handed (mutual of a friend introduced StepMania to us back in the day, and was playing stuff one handed on laptop arrow keys). Basically the equivalent of that is WASD, but on arrow keys in that shape. It is a bit odd, just using index / middle / ring finger across those three columns. You could play some older / easier stuff this way, but it was pretty limiting for certain patterns.

I've seen some more modern approaches to one handed which make use of the thumb, so like Space Bar on thumb and three other letter keys that you can comfortable hit with your index / middle / ring fingers.