How to Make It
In a large bowl, whisk the rice flour with the cornstarch, tapioca flour and salt. Whisk in the water and 2 teaspoons of the oil until blended.
In a small bowl, cover the tree ear mushrooms with warm water and let stand until softened, about 5 minutes. Drain and chop the mushrooms.
In a small skillet, heat the remaining 2 tablespoons of oil. Add the ground pork and the onion and cook over moderate heat, breaking up the meat with a spatula, until no pink remains, about 4 minutes. Stir in the chopped mushrooms and the fish sauce and season with salt and pepper.
Preheat the oven to 425°. Oil 3 large baking sheets. Heat an 8-inch nonstick skillet and brush with oil. Whisk the rice-flour batter well. When the oil is hot, pour 2 tablespoons of the batter into the skillet, tilting and shaking the pan to evenly coat the bottom with batter. Cover and cook over moderate heat until the crêpe is firm, about 2 minutes. With a spatula, flip the crêpe and cook for 30 seconds longer. Flip the crêpe out flat onto a prepared baking sheet. Repeat with the remaining batter to make a total of 18 crêpes; don't let the crêpes overlap on the baking sheets or they will stick together.
Spoon about 2 tablespoons of the mushroom filling into the center of each crêpe and fold in the sides to cover the filling and form a neat square. Cover the crêpes with foil and bake until heated through, about 5 minutes.
Arrange the rice crêpe packets on a platter and spoon the Nuoc Cham Sauce over them. Scatter the cucumber slices, fried shallots and bean sprouts all over the crêpes and serve.
Tapioca flour is available at specialty food stores. Fried shallots are available at Asian markets.