For the nuoc cham sauce: Pound the garlic and chile in a large mortar with a pestle until pasty. Whisk in the fish sauce, sugar, lime juice, and 1 cup water until the sugar dissolves. Transfer to a serving bowl for dipping.
For the banh xeo: Whisk the flour together with the scallion, salt, turmeric and 1/4 cup cold water in a bowl until completely smooth. Set aside.
Heat a well-seasoned 9-inch carbon steel skillet over high heat until smoking hot. Add enough oil to coat the bottom of the pan generously and swirl to lightly coat the sides, too. Add the shrimp and bacon, and cook, stirring and tossing, until the shrimp start to curl but before they become completely opaque, about 1 minute.
Whisk the batter again to mix in any flour that has settled on the bottom, then ladle a spoonful into the pan. Swirl the pan to coat the bottom and an inch or so up the sides with a thin layer of the batter. Repeat two more times. Let the batter set, about 30 seconds.
Drizzle the egg over the crepe and swirl the pan to evenly coat the crepe with egg. The crepe should have released from the sides of the pan; squirt oil between the crepe edges and the pan. Gently shake the pan to keep everything moving. Poke a hole in the crepe with a fork or spoon to keep it from bubbling up -- you want a flat crepe. Oil should be sputtering at the edges of the crepe; if it isn't, add more.
Cook the crepe, oiling and shaking until the egg has set on top and the bottom of the crepe is dark brown around the edges and speckled on the bottom. Pile the bean sprouts on one half and fold the other half over with a spatula. Hold the folded omelet against one side of the pan and tilt the oil out the other side into a heat-safe bowl; discard.
Lift the omelet out of the pan with the spatula and immediately serve with the shrimp and bacon, lettuce and shiso leaves, herbs and nuoc cham. Tear off a piece of omelet with a bunch of bean sprouts and tuck it into a lettuce leaf with a shiso leaf and some bacon and shrimp. Pluck a few fresh herb leaves of basil, cilantro, dill and mint, and stuff them in there, too. Then roll it up and dip into the nuoc cham. Eat. Repeat.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.