Opening last week, The Salsa Room (TSR) Baked Empanadas (menu details) serves saltenas, which are like empanadas, but also containing a liquid stew.  The Washington Post’s Tim Carman has a nice tutorial on how to eat a saltena here.  Baked TSR is the mobile component to their brick and mortar restaurant in Arlington, operating since 1991.  The truck is permitted in the District.