Whisk the rice flour, cornstarch, turmeric, and salt in a bowl. Gradually whisk in the coconut milk and water until smooth, then stir in the scallions. Rest the batter for 20 minutes.
Stir together the fish sauce, water, lime juice, sugar, garlic, and chili for the nuoc cham.
Cook the filling:
Heat a large nonstick skillet over medium-high heat. Add a little oil, then cook the pork until lightly browned. Add the shrimp and cook until just pink.
Push the filling to one side of the pan. Pour in a thin layer of batter and swirl the pan so it reaches the edges. Scatter bean sprouts over one half.
Crisp and serve:
Cover for 2 minutes, then uncover and cook until the edges are deeply crisp and the bottom is browned. Fold the crepe over the filling and transfer to a plate.
Repeat with the remaining batter and filling. Serve immediately with lettuce, herbs, and nuoc cham.
Notes
A hot pan and a thin batter are the secrets to crisp banh xeo.
Cook the crepes one at a time so steam does not soften the edges.
Serve the crepes immediately while the shells are at their crispiest.