Tex-Mex aaahh how I miss it. I lived in Austin, TX for 20 years. Now I live in FL and have yet to find any decent Mex or TX-MX food. I consider TX-MX to be: Mexican food with an American twist. I agree with the poster who said that it is "heavier" than real Mex.
TX-MX uses a lot of cheese - but REAL Tex-Mex uses queso quesadilla cheese; Asedero, and/or any blend of REAL Mexican-style cheeses - definitly NOT American cheese. The best Mexican Restaurants I know of offer the "real thing" AND "Tex-Mex".
Think about it - if someone were to ask you about American Food, what would you confine yourself to:
Corned Beef and Cabbage or Clam Chowder from New England?
BBQ from the South?
Spiedies from Upstate Southern-Tier NY?
You just can't classify an entire country's food.
Mexico has huge coastlines; I am sure that they eat a lot of seafood.
The interior is going to probably be more corn, bean, and beef/chicken based.
Anyway - My suggestions (haven't been to Austin in 3 years - may not be accurate)
Chuy's on Barton Springs Rd. - Now a small chain, but this is the original. Velvet Elvis paintings in one room, Old hubcaps from classic cars hanging from the ceiling in the other room - people stick their "Dots" on them with messages (too lengthy to go into here) Must Have: Green Chili Stew. Chuy's gets Hatch(New Mexico) Green Chilis every Fall and roasts them by the roadside in these wire barrels that they turn over an open flame.
"ChuyChangas" (Chimichangas) Are the best I have ever had - Meat, cheese, Mexican rice, and of course chilis in a flour tortilla and deep-fried. I ADORE their Green Chili Stew and have tried replicate it - nada. Their Happy Hour boasts drink specials (Margs&Mex beer)
and has a replica of a 50's Cadillac(?) trunk sticking out of the wall filled with containers of taco meat, beans, salsa, jalapeno-ranch dip, tortilla chips, Chili Con Queso (yes, I know, Velveeta and Rotel - so there is room for American cheese!!) etc....Historic place, this is the exact location where the Bush Twins got busted....Sorry such a long post. I've got more restaurants in Austin that I will post if anyone is interested