Combine the rice flour, coconut milk, cornstarch, water, scallions, turmeric and salt in a mixing bowl.
Mix well and let rest for 20 minutes (better if overnight).
Heat 1 teaspoon of the vegetable oil in a nonstick skillet, add ¼ of the pork, shrimp and white onion and cook for 30 seconds.
Stir the crepe batter then pour ¼ of it into the pan—tilting and swirling the pan to evenly coat the bottom with a thin layer of batter.
Sprinkle ¼ cup of the bean sprouts over the crepe and drizzle 2 teaspoons of the vegetable oil around the edges.
Cover the skillet and cook over medium heat until the bottom of the crepe is crispy and golden.
Using a spatula, fold the crepe in half to form a half moon and slide it onto a plate.
Keep warm while making the other three crepes.
Serve with the lettuce leaves, fresh mint and nuoc cham.