Place beef bones (marrow and knuckle) in a large stockpot and cover with cold water by 2 inches; bring to a rolling boil over high heat, 5β8 minutes, then drain, rinse the bones, and scrub the pot.
Halve the onion and smash the fresh ginger; arrange cut sides up on a foil-lined sheet and broil until deeply charred in spots, 8β12 minutes, turning once.
In a dry skillet, toast the star anise, cinnamon sticks, whole cloves, coriander seeds, fennel seeds, and black cardamom pod over medium heat until fragrant, 2β3 minutes; transfer to a spice sachet or set aside.
Return the cleaned pot and bones to the stove; add beef brisket, the charred aromatics, the toasted spices, rock sugar, fish sauce, and water. Bring to a bare simmer, then reduce to low and maintain a gentle bubble; skim foam and fat occasionally. Simmer until the meat is tender, 2.5β3.5 hours; lift out the meat and chill for neat slicing. Continue simmering the broth until deep and aromatic, 4.5β5.5 hours total; strain through a fine sieve and season with fine sea salt to taste.
Meanwhile, soak the dried rice noodles (banh pho) in hot tap water until pliable, 20β30 minutes; bring a large pot to a boil and cook the noodles just until tender, 30β60 seconds, then drain and briefly rinse.
Slice the cooled meat thinly across the grain and thinly slice the beef eye of round; place the white onion, scallions, cilantro, Thai basil, bean sprouts, limes, and Thai chiles on a platter for serving.
Bring the strained broth back to a rolling boil. Divide noodles among warmed bowls, add some sliced cooked meat and raw beef, and ladle 1.5β2 cups of boiling broth over each portion; the thin slices should turn just opaque in 10β20 seconds. Serve immediately with the platter of herbs and condiments, and pass hoisin sauce and sriracha at the table.