Cover the bones with water, bring to a boil for 5 minutes, drain, rinse, and clean the pot. Return the bones with 10 cups fresh water, onion, and garlic.
Simmer very gently for 2 hours, skimming as needed. Add the pork shoulder for the final 60 to 75 minutes, until tender. Remove the meat, cool slightly, and slice irregularly.
Strain the broth and season with fish sauce, sugar, and salt. Keep hot without boiling hard.
Prepare the Bowls:
Soak or cook the rice noodles according to package directions, then drain. Poach the shrimp in the hot broth for 2 to 3 minutes until just opaque.
Divide noodles among bowls. Add sliced pork, shrimp, bean sprouts, scallions, cilantro, and fried garlic. Ladle over hot broth and serve with lime.
Notes
Keep the stock at a bare simmer for a clear broth.
Store broth and garnishes separately for up to three days; reheat broth to a full simmer before serving.