Whisk rice flour, cornstarch, turmeric, salt, and sugar. Add water and coconut milk; whisk smooth. Stir in green onions. Rest at least 1 hour (or refrigerate overnight). Batter should be like heavy cream.
Dissolve sugar in warm water. Add fish sauce, lime juice, garlic, and chili. It should be salty first, sour second, lightly sweet. Adjust to taste.
Thin-slice pork; season lightly with salt and pepper. Peel and devein shrimp. Slice onion. Rinse and drain bean sprouts.
Heat a 10-inch nonstick or seasoned carbon steel skillet over medium-high. Add 1 tsp oil; sear a few shrimp, pork slices, and onion for about 1 minute until pork has color and shrimp start to pink.
Stir the batter; ladle in 1/3 cup, tilting and swirling to coat thinly. Scatter bean sprouts over one half. Cover; cook 1-2 minutes until edges lift and surface sets.
Uncover; drizzle 1 tsp oil around the edges. Cook 2-3 more minutes until deeply golden with lacy crisp edges. Fold in half and slide onto a plate. Wipe pan between crepes and repeat.
Tear a piece of crepe, lay on a lettuce leaf with herbs, roll loosely, and dip in nuoc cham.