Marinate & Mix: In a large bowl, roughly combine the cold ground pork, minced shallots, garlic, lemongrass, fish sauce, five-spice, and exactly half a package of the curing powder. Transfer to a stand mixer fitted with the paddle attachment. Mix on medium speed for 10–15 minutes until the meat turns into a highly tacky, sticky paste.
Build the Flavor: Transfer the paste back to a bowl, cover tightly, and let it marinate in the fridge for at least 2 hours (overnight is even better) to let those classic Vietnamese barbecue flavors develop.
The Ziploc Bag Hack: Slice open two sides of a plastic Ziploc bag so it opens up flat. Lightly brush the inside with a neutral oil. Using an ice cream scooper for consistency, place two scoops of meat paste near the center fold. Fold the plastic over. Using a tool with a straight edge (like a bench scraper or plastic ruler), press against the meat blob and glide it away from you to form a perfectly smooth, uniform log.
Air Fry: Slide the shaped sausages out of the plastic and arrange them in a single layer in your air fryer basket, leaving a little space between each so they brown perfectly. Air fry at 380°F for 10 minutes until deeply caramelized and cooked through. (If your air fryer only heats from the top, flip them halfway through).
Garnish & Serve: Plate the hot Nem Nướng and immediately brush with warm scallion oil to lock in the juiciness. Top with a sprinkle of crushed roasted peanuts.