1, To make the batter, in a large bowl, whisk together the rice flour, tapioca starch, potato starch, salt, and 5 cups water. Cover and refrigerate overnight. The following day, pour off all of the water from the bowl (it should measure about 2½ cups) and replace with an equal amount of fresh water. Stir until the water is incorporated and the batter is smooth. Set aside.
2, To make the filling, in a bowl, combine the pork, 2 tablespoons of the oil, 1 tablespoon of the fish sauce, the sugar, cornstarch, and pepper and stir to combine. Let marinate at room temperature for 15 minutes.
3, In a large sauté pan, heat the remaining ½ cup oil over medium heat. Add the garlic and sauté for 15 seconds. Add the onions and the pork mixture and cook, breaking up the pork with a wooden spoon, for about 5 minutes. The pork should not brown, so lower the heat if necessary. Add the water chestnuts and salt and continue cooking for 2 minutes longer, until the pork is no longer pink and the onions are soft. Add the mushrooms and stir just to combine, then season the mixture with the remaining 1 tablespoon fish sauce. Remove from the heat and let cool completely. (The filling can be made up to 2 days in advance, cooled, covered, and refrigerated. Bring to room temperature before using.)
4, Set up your bánh cuón production line: Fill a bánh cuón pot (or your makeshift pot) with water and bring to a boil over medium-high heat. Oil a rimmed baking sheet with canola oil and set aside. Oil a dinner plate and place in close proximity to the bánh cuón pot. Set additional oil and a pastry brush nearby so you can oil the plate as needed. Have your pork filling at the ready.
5, Ladle ¼ cup of the batter into the center of the fabric on the pot and, using the bottom of the ladle, quickly and gently spread the batter into a thin, even circle about 8 inches in diameter. Immediately cover the pot and let steam for 1 minute. When you uncover the pot, the rice paper should look set. Do not cook it too long or it will begin to crack.
6, Slide a 12-inch-long offset cake spatula or bamboo stick under the edge of the rice paper farthest from you and use your thumb to anchor the center of the rice-paper sheet on the stick. Pull the stick toward you in one smooth motion, pulling the rice-paper sheet free. Transfer it to the greased plate and straighten it as needed so it is flat and wrinkle free.
7, Spoon 2 tablespoons filling in the center of the rice-paper circle, spreading it horizontally into a rectangle about 4 inches long by 1 inch wide.
8, Fold in the sides of the rice paper, creating right angles, then fold the bottom up and over the filling. Fold the top of the circle down so that the top edge touches the filling, then fold it a second time so the top edge of the rice paper comes up and over the filling, enclosing it completely and forming a tight little package.
9, Transfer the roll to the prepared baking sheet. Continue making and filling the rice-paper sheets until you have used all of the batter and filling, greasing the plate as needed between batches and replenishing the water in the pot as necessary.
10, To serve, line a platter with the shredded lettuce, mint, and cucumber and top with the rolls. Garnish with the shallots and serve the fish sauce alongside for dipping.