Make the batter: In a large bowl, whisk together rice flour, coconut milk, water, turmeric, salt, and 2 chopped green onions until smooth. Let rest for at least 30 minutes.

Prepare the filling: Heat 1 tbsp oil in a pan over medium-high heat. Add sliced pork and cook until browned. Add shrimp and cook until pink. Add sliced yellow onion and cook until softened. Stir in 2 chopped green onions, bean sprouts, fish sauce, sugar, and pepper. Cook for 2-3 minutes until bean sprouts are slightly wilted. Remove filling from pan and set aside.

Cook the Banh Xeo: Heat 1-2 tbsp oil in a non-stick pan or well-seasoned cast iron skillet over medium-high heat. Pour about 1/2 cup of batter into the hot pan, tilting to spread it thinly and evenly. Immediately add a portion of the cooked filling (shrimp, pork, bean sprouts) to one half of the crepe.

Cover and Crisp: Cover the pan for 2-3 minutes to steam the crepe and cook the filling through. Uncover, and continue to cook for another 3-5 minutes, or until the edges are golden brown and crispy.

Fold and Serve: Carefully fold the crepe in half with a spatula. Slide onto a serving plate. Repeat with remaining batter and filling.

Make Nuoc Cham (Dipping Sauce): In a small bowl, combine fish sauce, water, sugar, and lime juice. Stir until sugar dissolves. Add minced garlic and chilies (if using).

Serve: Serve Banh Xeo hot with fresh lettuce leaves and Nuoc Cham dipping sauce.