Make the batter: In a large bowl whisk together the Rice flour, Cornstarch, Ground turmeric and Salt. Gradually whisk in the Coconut milk and Water until smooth and thin (consistency of heavy cream). Stir in the sliced Scallions and let the batter rest 15–30 minutes.
Prep the filling: Thinly slice the Pork belly, peel and devein the Medium shrimp, and thinly slice the Yellow onion. Season the pork and shrimp lightly with salt and freshly ground Black pepper.
Cook the proteins: Heat a large nonstick or cast-iron skillet over medium-high heat and add a splash of Neutral oil. Sear the pork belly until browned and slightly crisp, transfer to a plate, then cook the shrimp until just opaque and set aside. In the same pan sauté the onion until softened and slightly caramelized; combine onion with the pork and shrimp and keep warm.
Cook the pancakes: Return the skillet to medium-high and add another teaspoon of Neutral oil and swirl to coat. Pour in a ladleful of batter, tilting the pan to spread it thinly into a round. Quickly scatter some of the cooked pork and shrimp and a handful of Bean sprouts onto one half of the crepe.
Finish the pancakes: Let the crepe cook until the edges are lacy and the bottom is deeply golden and crisp, 2–4 minutes; cover briefly if the top is still very wet to steam and set. Fold the pancake in half and slide onto a platter. Repeat with remaining batter and filling, adding oil as needed.
Make the dipping sauce (nuoc cham): In a small bowl dissolve the Granulated sugar in warm water, then stir in Fish sauce and Lime juice to taste. Add minced Garlic and thinly sliced Bird's eye chilies; adjust sweet, salty and sour balance until bright and harmonious.
Serve: Arrange the pancakes with a platter of Butter lettuce and mixed herbs—Mint, Cilantro and Thai basil. To eat, pull off a lettuce leaf, add a piece of pancake and herbs, wrap, and dip into the nuoc cham.
