Hmmm....I joined a couple of pals a few months ago at a LaFinca in far north Dallas..Plano/Frisco area. The food was very good there. I wonder if it is the same restaurant...??
Anyway, I think you're referring to a Salsa Verde, which this restaurant served with corn chips, along with regular salsa and pica de gallo. To make Salsa Verde...
Combine in a blender:
1 pound tomatillos
2 small chiles
1/4 cup green onions, chopped
1 cup fresh cilantro
1 clove garlic, peeled
When it's nice and smooth, add more salt, if necessary, and about 1/2 teaspoon of sugar.
Pain is inevitable. Suffering is Optional.