Make the Crepe Batter: In a large bowl, whisk together rice flour, coconut milk, turmeric powder, salt, and water until smooth. Stir in sliced scallions. Let sit for 15-20 minutes to allow the flour to hydrate.

Prepare the Nuoc Cham: In a small bowl, combine fish sauce, sugar, warm water, lime juice, minced garlic, and sliced chili (if using). Stir until sugar dissolves completely. Set aside.

Cook the Filling: Heat 1 tablespoon of vegetable oil in a large non-stick skillet or wok over medium-high heat. Add sliced pork belly and cook until browned and crispy. Add shrimp and onion, cooking until shrimp turn pink and onion softens. Season with 1 tablespoon fish sauce and black pepper. Remove the cooked filling from the skillet and set aside.

Cook the Crepes: Wipe the skillet clean if necessary. Heat 1 tablespoon of fresh vegetable oil over medium-high heat until shimmering. Ladle about 1/2 cup of batter into the hot skillet, immediately tilting and rotating the pan to spread the batter thinly and evenly around the bottom and slightly up the sides.

Assemble the Crepes: Immediately scatter a portion of the cooked pork, shrimp, and bean sprouts over one half of the crepe. Cover the skillet with a lid for 1-2 minutes to steam the bean sprouts slightly and ensure the crepe sets.

Fold and Serve: Remove the lid. Cook until the edges are crispy and golden brown, and the center is set and cooked through (this usually takes about 3-5 minutes total per crepe). Fold the crepe in half using a spatula. Carefully slide the finished Bánh Xèo onto a serving plate. Repeat with the remaining batter and filling, adding more oil as needed for each crepe. Serve hot with fresh lettuce, an assortment of fresh herbs (mint, Thai basil, cilantro), and the prepared Nuoc Cham dipping sauce. To eat, tear off a piece of crepe, wrap it in lettuce and herbs, then dip in the sauce.