Whisk the rice flour, turmeric, and salt in a large bowl. Gradually add the coconut milk and water, whisking until the batter is smooth and has the consistency of heavy cream, then stir in the scallions.

Let the batter rest for at least 30 minutes at room temperature—this allows the rice flour to hydrate so the crêpes don't crack.

Heat a teaspoon of oil in a 10-inch nonstick skillet over medium-high heat until the oil shimmers. Add a portion of the mushrooms and onions, cooking until they are softened and just beginning to brown.

Quickly stir the batter to recombine, then pour a half-cup of batter into the skillet while swirling the pan gently to distribute it evenly over the bottom. The batter should sizzle immediately and form small holes.

Cover the skillet with a tight-fitting lid and cook until the edges of the crêpe are set and starting to pull away from the sides, about 2 minutes.

Remove the lid and scatter a handful of bean sprouts over half of the crêpe. Continue cooking uncovered until the bottom is deeply golden and very crispy.

Fold the crêpe in half over the sprouts and slide it onto a plate. Repeat the process with the remaining oil, vegetables, and batter, wiping the skillet clean between batches if necessary.

Serve immediately while the edges are still crackling, accompanied by lettuce leaves, herbs, and lime wedges for wrapping.