This traditional Vietnamese recipe is a favorite of Mai Pham, chef and owner of the Lemon Grass Restaurant in Sacramento, California.
Make the batter: Combine rice flour, water, oil, and salt in a bowl; whisk until smooth. The batter will be thin and watery. Strain to remove any lumps, and let stand for at least 30 minutes.
Make the filling: Heat oil in a skillet over medium heat. Add onion and shallots, and cook, stirring, for 2 to 3 minutes. Add mushrooms, shrimp or ground pork, fish sauce, sugar, and white pepper, and cook until shrimp turns pink, 2 to 3 minutes. Transfer to a bowl, and set aside.
Bring the rice-roll pot (make sure it's been filled halfway with water) to a boil. Oil a baking sheet, and place it near the stove along with the filling.
Make the rice sheets: Stir the batter well before making each rice sheet. Using a shallow ladle, spoon 2 tablespoons batter onto the center of the cloth. Using the bottom of the ladle, spread the batter to make a 6-inch-wide round sheet. Be sure to work quickly, and try not to put pressure on the cloth. Cover, and steam until sheet is translucent, about 40 seconds. Uncover. Hold a thin spatula or bamboo stick at a 45-degree angle and push gently against the side of the rice sheet. When about 1/2 inch has gathered on the spatula, lift the sheet off in a steady motion and transfer to the oiled baking sheet. (Do not stack the sheets.) Gently scrape the cloth clean, and repeat. After cooking several rice sheets, tighten the cloth by pulling on the sides. Refill the pot with hot water as necessary by pouring it through one of the slits in the cloth.
Let the rice sheet cool for 30 seconds. To fill the rice rolls, make a 1-inch fold from the bottom edge of the rice sheet. Neatly place 1 1/2 teaspoons filling on top of this double layer. Fold the sides over, and roll into a cylinder about 2 1/2 inches long and 2/3 inch wide. These can be made in advance up to this point and stored in the refrigerator for 2 days. Before serving, reheat in a microwave oven until just warm, not hot.
To serve, divide the rice rolls among four plates. Top each plate with one quarter of the bean sprouts, mint, perilla or basil, and shallots. Drizzle with sauce, and serve immediately.