The chef and cookbook author explains why some of Mexico’s most iconic dishes — from chiles en nogada to torta ahogada — remain unfamiliar to many American diners.Credit: Food & Wine / Getty ImagesWhen I ask chef and cookbook author Rick Martínez if there’s one Mexican dish he believes people overlook, I expect him to champion a regional specialty or a personal favorite. Instead, he gently rejects the premise of the question.“As far as overlooke…