Season pork with salt and pepper. Heat 2 tbsp. of oil in a wok or large non-stick skillet, over med-high heat. Cook pork strips, until browned 1-2 minutes, each side. Make sure your pan is very hot so you get a nice sear on the outside of your pork. Be careful not to overcook the pork or it will get chewy. Transfer to a plate and reserve. Add 2 tbsp. oil to skillet. Working in batches cook onions, celery, carrots, bok choy stems, cabbage, scallions, garlic until just crisp tender, about 2 minutes per batch. Reserve vegetables on plate with pork.
Add lemon juice to skillet, bring to a simmer until almost completely evaporated. Add stock and cook until liquid has reduced by half. Season broth with pinch of salt (or soy sauce), pepper and fish sauce.
Meanwhile, quickly soak your rice noodles in warm water. This is the tricky part because rice noodles are delicate. If you soak them in boiling water or over soak them, your noodles will become mushy. The idea is to soak the rice stick noodles just long enough to separate them and then immediately drain and run cold water over them to stop the cooking.
Add noodles to stock, tossing constantly (tongs are very helpful here) until noodles are just cooked through and liquid has been absorbed. Add reserved pork, vegetables, bok choy leaves and juices accumulated on plate and toss until evenly distributed and warmed through. Remove from heat. Taste and adjust season with more salt (or soy), pepper, lemon juice or fish sauce. Transfer to a serving platter. Top with celery leaves, scallion greens and lime wedges.