There is currently no such thing as a recumbent suspension mountain bike, at least not on a mass-produced scale. The rigors and demands of mountain biking would be nearly impossible to deal with from a recumbent position. This makes the idea of this type of bike a bit impractical.
Now that being said, there are people that have designed their own recumbent suspension mountain bike. As a general rule, these bikes are street-use recumbents that have been modified by their owners. There are, however, a few companies out there that are experimenting with varying off-road specific designs.
If you choose to make your own recumbent suspension mountain bike, you will need to make some decisions as to the modifications needed. First, while many recumbents use a suspension system, for off-road use you will definitely need to beef up this part of the bike. Other modifications should include tires and an upgraded gearing system.