Make the batter: place the rice flour, coconut milk, water, turmeric, sugar, salt, curry powder and scallions in a bowl and stir well to blend. Set aside.
Make the filling: Heat 1 tablespoon of the oil in a large nonstick skillet over high heat. Add 1/4 each of the onion, the pork and the shrimp and stir until fragrant, about 15 seconds. Whisk the batter well, and ladle about 2/3 cup into the pan. Swirl so the batter completely covers the surface. Neatly pile about 1 cup bean sprouts and 1/2 cup mushrooms on one side of the crepe, closer to the center than to the edge. Reduce the heat slightly, cover the pan and cook until the edges pull away from the sides of the pan, about 5 minutes. Reduce the heat to low. Uncover and cook until the crepe is crisp and the pork and shrimp are done, another 2 to 3 minutes. Slip a spatula under the crepe to check on the bottom of the crepe. If it?s not brown, cook for another 1 to 2 minutes.
Lift the side of the crepe without the bean sprouts and mushrooms and fold it over the covered side of the crepe. Using a spatula, gently slide the crepe onto a large plate. Wipe the pan clean and make the remaining crepes in the same way. Be sure to oil the pan before beginning the next crepe.
To serve, place the crepes, Vietnamese Dipping Sauce and some lettuces, mustard leaves, and herbs on the table. To eat, tear a piece of the banh xeo and wrap the lettuce or mustard leaves and herbs. Roll into a packet, then dip into the sauce and eat.
Vietnamese Dipping Sauce: Nuoc Cham
Cut the chiles into thin rings. Remove one-third of the chiles and set aside for garnish. Place the remaining chiles, garlic, and sugar into a mortar and pound into a coarse, wet paste. Transfer to a small bowl and add the water, lime juice, and fish sauce. Stir well to dissolve. Add the reserved chiles and carrots. Set aside for 10 minutes before serving.
Recipe courtesy Mai Pham from Pleasures of the Vietnamese Table HarperCollins 2001