Sure you can do good photos with just one flash. Look at some examples from last night’s Sheridan College class.

One flash, fitted with a Honl photo 1/4″ grid:

One flash, fitted with a small Honl photo 12″ softbox:

And one flash, fitted with a shoot-through umbrella:

As you see, all these are acceptable or good. The umbrella is a little softer, but it throws light all over the room. The softbox is probably the best option here.

I used the standard “studio settings”: 1/125 sec, f/8, at 200 ISO in order to keep the ambient light out.