Need to add an outlet under a sink (for garbage disposal)
Surface mount it through the cabinet back to a stud

If you can find a stud in the wall, it'd be best to run the NM into a surface mounted box (a "handy box" will do) that's screwed through the cabinet back and wallboard to a stud, then wire up the receptacle, mount it to the "handy box", and put a cover on it.  (You can get away without a ground pigtail if you use a self-grounding type receptacle and a one-hole box cover, BTW.)