1. **Prepare the Batter:** In a large bowl, combine rice flour, all-purpose flour, turmeric powder, and salt. Gradually whisk in the coconut milk and water until you have a smooth, lump-free batter. Stir in the chopped green onions. Let the batter rest at room temperature for at least 30 minutes (or up to 2 hours) to allow the flours to hydrate fully.

2. **Prepare the Filling:** Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add the thinly sliced pork belly and cook until browned and lightly crispy, about 5-7 minutes. Remove pork and set aside.

3. Add the shrimp to the same pan and cook until pink and opaque, about 2-3 minutes. Remove shrimp and set aside.

4. Add the remaining 1 tablespoon of oil to the pan. Add the thinly sliced onion and sauté until softened, about 3-4 minutes. Stir in the bean sprouts and optional cooked mung beans, cooking for just 1-2 minutes until slightly softened but still crunchy. Return the cooked pork and shrimp to the pan. Season the filling with salt and pepper to taste. Mix well and set aside.

5. **Make the Nuoc Cham Dipping Sauce:** In a small bowl, combine fish sauce, sugar, warm water, and lime juice. Stir until the sugar dissolves. Add the minced garlic and sliced chili (if using). Taste and adjust seasoning as needed.

6. **Cook the Banh Xeo:** Heat a 22cm (8.5-inch) non-stick pan over medium-high heat. Add a tablespoon of vegetable oil, swirling to coat the bottom and sides. The pan needs to be hot enough for the batter to sizzle immediately.

7. Give the batter a quick stir before each crepe. Pour about 1/2 cup of batter into the hot pan, quickly tilting and rotating the pan to spread the batter thinly and evenly over the entire surface, creating a thin, lacy crepe.

8. Immediately add a portion of the pork and shrimp filling to one half of the crepe. Cover the pan with a lid for 1-2 minutes to steam the filling and cook the batter slightly.

9. Remove the lid. Cook for another 3-5 minutes, or until the edges of the crepe are golden brown and crispy. The bottom should be deeply golden and crisp. Use a spatula to gently loosen the crepe from the pan. Fold the crepe in half over the filling.

10. Carefully slide the crispy Banh Xeo onto a serving plate. Repeat with the remaining batter and filling, adding a little more oil to the pan before each new crepe.

11. **Serve:** Serve the hot Banh Xeo immediately with large lettuce leaves, fresh herbs (mint, basil, cilantro), and the Nuoc Cham dipping sauce. To eat, tear off a piece of crepe, wrap it in a lettuce leaf with fresh herbs, and dip into the sauce.