I need a recipe for chili relleno sauce.

I love chili rellenos, and my daughter gave me a wonderful recipe for them that is easy and works every time. But what is that red sauce that they are served with in the restaurants? It isn’t salsa, or mole. I have tried both and that Is definitely not it. I tried just Mexican style tomato sauce – not it either. It is kind of thin, and oily. What is it? They also serve this sauce with tamales.