In a stock pot, cover oxtail and brisket with cold water and bring to a boil; then drain and discard the water.
Repeat the process one more time, then rinse meats and return to the (clean) pot; add ginger, garlic, and onions.
In a cast iron pan, combine cinnamon sticks, black peppercorns, and star anise and toast over high heat, shaking the pan occasionally, until the spices are fragrant. Add the spices to stock pot.
Fill the pot with about 5 quarts (litres) cold water, enough to cover everything by 1 inch (2.5 cm).
Bring stock to a simmer over high heat, then reduce heat to low and simmer (do not boil) until the brisket until it is tender, approximately two to three hours. Skim off and discard any scum or fat that floats to the top as needed.
When the brisket is tender, remove it and set it aside to cool completely.
Continue to simmer the broth and oxtail for four hours more, until oxtail meat is tender and falling from the bone.
Strain and separate the solids from the broth. Reserve the broth in a clean pot over medium to medium-high heat. Separate the meat from the oxtail and reserve; discard the bones and aromatics.
Skim any remaining fat from the broth, then stir in the fish sauce and yellow rock sugar.
Thinly slice the chilled and cooked brisket and the raw sirloin.
Divide the noodles among six to eight large bowls, top with the brisket, sirloin, and the oxtail meat.
Ladle the very hot broth over the noodles and meat and serve immediately.
Serve with (optional) garnishes on side: bean sprouts, onion, jalapeño pepper, Thai basil, lime wedges, hoisin sauce, and sriracha.