Mexican Cheese Sauce?
That sounds like a standard queso cheese (typically found on your chimis etc.)

If you are looking for a terrific queso recipe, there are myriad. However, two tips for any queso: 


seek out "Chihuahua Cheese" (its a mild white that melts very smoothly and has a subtle flavor)
pepper and cumin and paprika roasted first are your friends, but if you add more than a little, they are your worst enemies. (also, fresh roast jalapenos for ideal flavor, don't use diced.)