1 small light pointed at the background, zoomed in or snooted + 1 main light off to the side which is flagged or gobo'd to prevent it spilling onto the background.
You don't need a dark grey background, a plain white wall will do, the darkness comes from limiting the power of the light pointed at the background. If you have a white background you can make it any colour you like by placing a gel over the flash.
Here is a similar effect but using a curtain as the background:
The shot on the left is with only the main light on, the shot on the right is with only the background light. Notice how none of the main light is hitting the background, and none of the background light is hitting the subject.
This is good practice as it gives you total control over the balance of lights. Here's the setup:
You can see the main light on the left, a coat hanging from a lightstand is preventing it from hitting the background (you have to use whatever you have to hand). The background light is on the right with a snoot providing a tight spot.