In a large pot, combine the pork bones, water, onion, garlic, and ginger.
Bring to a boil, then reduce heat and simmer for 1-2 hours. Skim off any foam that forms on the surface.
After simmering, strain the broth through a fine-mesh sieve to remove the bones and vegetables. Season with fish sauce and salt to taste.
Cook the rice noodles according to the package instructions. If using dried noodles, soak them in warm water for 20-30 minutes before cooking. Drain and set aside.
Heat the vegetable oil in a large skillet or wok over medium heat.
Add the thinly sliced pork and cook until just done, about 3-4 minutes. Remove from heat.
In serving bowls, place a portion of cooked rice noodles.
Top with cooked pork, bean sprouts, fresh herbs, and green onions.
Ladle the hot broth over the noodles and toppings.
Enjoy your meal!