Because these crêpes must be cooked one at a time, invite guests to start eating as soon as the crêpes come out of the skillet. To eat, cut off pieces of the crêpe, fold them along with the herbs in the romaine leaves, and dip in the nuoc cham.
Whisk to combine in a medium bowl:
Set aside while you prepare the remaining ingredients. Prepare and lay out on a large plate or rimmed baking sheet:
Heat in a large nonstick skillet over medium-high heat:
Add one-quarter of the pork and onion and cook, stirring, until the pork is no longer pink, about 1 minute. Add one-quarter of the shrimp and spread the pork, shrimp, and onion evenly in the skillet. Pour ½ cup of the batter into the skillet and quickly tilt and rotate the skillet to coat it with batter. Cook the crêpe until it just appears set, about 1 minute. Scatter ½ cup of the bean sprouts on half of the crêpe, and cover the skillet. Cook until the sprouts are slightly translucent, about 1 minute. Uncover the skillet and drizzle around the edge of the crêpe so it runs beneath the crêpe, not on top of it:
Continue to cook until the crêpe is crispy, 1 to 2 minutes more. Fold the crêpe in half in the skillet to cover the bean sprouts, then slide it onto a plate. Repeat this process three more times with the remaining fillings, batter, and more oil. Serve the crêpes with: