Authentic Mexican within 15 miles of Scotch Plains?
How about Mis Amigos in Dunellen? It's very authentic and welcoming.
373 North Ave
Cinco de Mayo in New Brunswick (maybe a tiny bit more than fifteen miles) - don't be put off by the exterior! It's fine inside.
Cinco de Mayo
206 French St
When I drive through Plainfield, there always seem to be a few promising looking places, but I never get the chance to stop. Perhaps there's something worthwhile there too.
I was going to say almost any place in Bound Brook, New Brunswick or Plainfield. My usual is Crazy Burrito, but picking a random place in one of the above towns usually pays off. It's also worthwhile going Central American instead of Mexican. There are a handful of Honduran/Dominican/Guatemalan/etc places.
Double-tortilla tacos are the only example I can think of where authentic isn't as good as Americanized.