01 - Whisk together rice flour, tapioca starch, cornstarch, salt, water, and vegetable oil in a bowl until completely smooth and lump-free. Let the batter rest for at least 20 minutes to achieve proper consistency.
02 - Heat 1 tablespoon oil in a skillet over medium heat. Sauté onions and garlic until fragrant. Add ground pork and cook until browned. Stir in chopped wood ear mushrooms, fish sauce, and pepper. Cook for 2-3 minutes until fully cooked and fragrant. Remove from heat and set aside.
03 - Combine fish sauce, lime juice, sugar, and warm water in a small bowl, stirring until sugar completely dissolves. Add minced garlic and sliced chili. Taste and adjust seasoning as needed.
04 - Bring water to a boil in a wide steamer or large sauté pan fitted with a nonstick, lightly oiled cloth or silicone tray. Ladle approximately 1/4 cup batter onto the surface, swirling quickly to coat evenly. Cover and steam for 1-2 minutes until set and slightly translucent.
05 - Gently transfer the cooked sheet onto a lightly oiled plate using a thin spatula or offset spatula. Place a spoonful of filling along one edge and roll up tightly. Repeat with remaining batter and filling.
06 - Arrange completed rolls on a serving platter. Top generously with fried shallots, cilantro, bean sprouts, cucumber, and sliced pork sausage if desired. Serve immediately with nuoc cham dipping sauce on the side.