In a bowl, whisk together the onions, sugar, soy sauce, oil, garlic, fish sauce, lemongrass, and pepper.
Thinly slice the pork shoulder into 1/2 in slices, and then cut crossways into 4-inch pieces.
Add the pork to the marinade and let it rest for 30 minutes or for up to 24 hours.
Remove the pork slices from the marinade, and place in single layer in the air fryer basket.
Set the air fryer to 400ºF and cook the pork for 10 minutes, flipping over halfway.
Test with a meat thermometer to ensure the pork has reached an internal temperature of 165ºF.
Remove pork to a serving tray, sprinkle with the roasted peanuts and cilantro or parsley, and serve.
I bought a tube of lemongrass paste at my grocery store (in the produce section) but you can always either mince up some fresh lemongrass or omit if you can't find it.