Blanch bones: cover bones and brisket with cold water, bring to a boil for 10 minutes, then drain and rinse to remove scum.
Char aromatics: char onion and ginger over a flame or under the broiler until blackened; rinse lightly.
Toast spices: in a dry skillet toast star anise, cloves, cinnamon, coriander and fennel for 2–3 minutes until fragrant.
Simmer stock: return bones and brisket to a large pot, add 8 quarts water, charred aromatics and toasted spices; bring to a gentle simmer.
Skim often: simmer gently and skim foam every 15–20 minutes to keep the broth clear.
Add seasoning: add rock sugar and fish sauce after 1–2 hours; continue simmering low for a total of 3–4 hours (longer for deeper flavor).
Remove brisket: after 1.5–2 hours remove brisket, cool and thinly slice against the grain.
Strain broth: strain through a fine sieve or cheesecloth into a clean pot, discarding solids.
Prepare noodles: cook or soak rice noodles according to package instructions just before serving; drain and rinse with hot water.
Assemble bowls: place noodles and sliced brisket or raw thin eye round in bowls; ladle boiling broth over to gently cook raw beef, then top with herbs, scallions, bean sprouts and lime.