Mix the pork and minced shrimp in a large bowl, then add the egg and stir to combine. In a separate bowl, stir together the soy sauce, salt, white pepper, and five-spice powder until smooth, then add it to the pork and shrimp mixture.
Stir in the water chestnuts, green onions, and yellow onion, mixing to distribute the ingredients evenly. Finally, sift in the flour and mix until no traces of flour remain.
Lay out the prepared skins on your work surface. Arrange a heaping tablespoon of the pork mixture along the longer edge of the skin, leaving a ½-inch gap from the edges. Shape the meat into a slim sausage, approximately 1 inch (2.5 cm) tall and 1½ inches (3.5 cm) wide. Add more pork if needed.
After shaping the meat, roll the skin starting from the edge closest to you, tucking in the side edges as you go. Roll until the meat is fully encased within the skin, then place it seam side down on a plate. Repeat until you’ve used up all the pork. (For a helpful step-by-step visual, check out The Little Teochew's images of how to roll ngoh hiang check out at the end of this post.) Lightly grease a steamer tray and steam the rolls for 8 to 10 minutes, until the skins turn translucent and the rolls feel firm. Remove them and set aside on wire racks to cool. At this point, you can divide the rolls into batches and freeze them in plastic wrap for up to 3 months. To cook, defrost the rolls in a 350°F (180°C) oven for 10 minutes before frying.
To finish the rolls, heat a non-stick saucepan large enough to comfortably hold 2 to 3 rolls, and add enough oil to thinly cover the surface of the pan.
Add the rolls one at a time—cooking a maximum of 3 at a time to avoid overcrowding the pan—and fry on medium-high heat until the skins turn a crisp dark brown. Allow them to cool on paper towels or a wire rack before slicing and serving. Wipe down the pan with paper towels after each batch before cooking the next.
Slice the rolls into 1-inch pieces and serve warm or at room temperature with the dipping sauce.