aka 牛肉(beef) 捲(rolled) 饼(pancake)
Since it's on my list, I figured I'd do a quick survey post on the dish.
This has become a pretty popular cult dish in Southern California and elsewhere. In the US it's usually seen sliced up as an appetizer. I've got an inkling that it is served intact as a street food sandwich in China, like its cousins the mighty popiah (aka run bing or bao bing, this is a Fujian 'crepe' that is also popular in Malaysia and Singapore. It features a wide variety of fillings and flavorings. It's sauced with a smear of a sweet bean based sauce, and optionally chili sauce.) and the tasty jian bing (A Bejing crepe with egg cooked on one side. Minimally served with scallions and a smear of chili sauce. Can also have a piece of fried dough and/or veggies as a filling).
-Flatbread. Varies, but the best are crispy, sorta flaky, and chewy. Should ideally be made fresh. Seems like most places use either a plain or sesame shaobing type of bread.
-Meat. I haven't dug up any 'official' recipes yet, but it seems to be some sort of braised beef with soy and 5 spice flavor. When I attempt this later, I will probably use some char sui or thit nuong (Chinese and Vietnamese bbq pork respectivly), because I've been meaning to do that this week. Longs like I will technically end up with Zho Rou Juan Bing.
-Sauce. Use a little of either sweet bean sauce aka sweet flour sauce (tian mian jaing), or hosin sauce. Siracha optional.
-Vegetables. Always have scallion. Other good additions are cilantro, bean sprouts, lettuce, shredded carrots, slice jalapeno. Go wild.