I think Ostioneria 7 Mares, at 3317 W. Walnut St. in Garland, qualifies as a hole in the wall.
For those old enough to remember, the building was originally a Bonanza Steakhouse
They serve TexMex seafood, and traditional Mexican dishes.
http://7maresostioneria.com
Del's burgers 110 S. McKinney. Richardson. About a block from Belt Line / Greenville
http://www.delsburgers.com/
Babe's is not so much a hole in the wall, but they make damn good chicken. I had it yesterday. They have 9 locations but quite a few people think the Garland location is best
http://www.babeschicken.com/our-kitchens/garland/