Place the pork, onion, and sauce into the slow cooker, and mix up the ingredients a bit so that the pork is coated in sauce. Cook on low for 8 hours. Shred the pork using two forks.
Making scallion pancakes (reference: Serious Eats):
Add flour to a food processor’s bowl. Gradually add boiling water while running the processor until the dough comes together and rides around the blade. Take the dough out, knead it a few times to form into a ball, and place in a bowl covered with a wet paper towel. Let sit for at least 30 minutes at room temperature.
Divide the dough into 4 round lumps. For each, roll into an 8-inch flat patty, brush the surface with some oil, sprinkle scallions, roll up, twist into a tight spiral, and flatten and re-roll into an 8-inch circle again. By re-rolling, this creates layers in the scallion pancake.
Heat some oil in a large pan over medium-high heat. Cook each scallion pancake until both sides are golden brown, about a few minutes per side. Transfer to a paper towel to dry.
Assembly:
Distribute the pulled pork (with onions and sauce) amongst the 4 cooked scallion pancakes. To eat, fold the scallion pancake like a taco.