The secret to the wonderful flavor of this sauce is the tomatillo, a fruit generally known as the Mexican green tomato. There is no substitute for the tomatillo, since ordinary unripe tomatoes don't have the same characteristics. Originally from the R.S.V.P. section of a November 1978 Bon Apetite magazine. The recipe request was from Don Hernando's restaurant in the Beverly Wilshire Hotel in Beverly Hills, Ca.