Put the mushrooms in a bowl, cover with water and soak for 20 minutes, then drain and thinly slice. To make the batter, combine rice flour, tapioca flour and salt with 20 fluid ounces/ 600 ml cold water. Now whisk until the flour dissolves and forms a smooth batter. Heat a nonstick frying pan over medium heat, then add 2 tablespoons oil. Fry the garlic and shallots until fragrant, then add the pork, mushrooms, fish sauce, sugar, and pinch of salt and pepper. Stir-fry for 4 minutes, then transfer to a bowl and set aside. Brush a round tray with oil and place beside the stove. Heat a large nonstick frying pan over medium heat, and add enough oil to coat the base of the pan. Pour a small ladleful (2 to 3 tablespoons) batter into the pan, turning the pan in a circular motion to cover the base with a thin layer of batter. Cover the pan with a lid and cook for 30 seconds. Remove the lid and slide the thin noodle sheet onto your oiled tray. Scoop 1 tablespoon pork mixture onto the noodle sheet, fold 2 sides in, then fold over to form a roll. Transfer to a plate. Repeat this process using the remaining batter and pork mixture, adding oil to the pan as necessary. Top the rolls with pork terrine, fresh herbs, bean sprouts, fried shallots, and cucumber. Dress with nuoc cham and serve with sliced chile.