Bring a small pot of water to the boil and blanch the oxtails for 5 minutes. Bring a second, larger pot with 8 cups of water to a boil.
Transfer the oxtails to the second pot along with the charred onion, charred ginger, and 8 cups of water. Simmer on low for 3.5 hours.
Char pho spice mix, then add to the pho soup along with the brisket and continue simmering for another 2 hours (5.5 hours total)
Transfer the brisket to its own container, then strain and separate the pho soup. Refrigerate both.
Skim the solidified fat from the pho soup with a fork and melt over low heat, then strain and refrigerate in a new small container. Prepare pho toppings as needed, cover and refrigerate.
When you are ready to serve
Combine the pho soup with enough water to make 8 cups, then season with fish sauce, sugar, and salt, if needed. I like 1 tsp of fine sea salt, but you should feel that your pho is on the verge of being too salty to compensate for the unsalted meat and noodles. Bring the soup to a low simmer. Slice the cold brisket and drop into the soup to reheat.
Bring a large pot of water to the boil and cook the noodles halfway, then rinse in cold water and drain. Heat up the bowls with hot tap water for 5 minutes, the drain. Divide the pho noodles evenly between bowls.
Slice sirloin if necessary, then divide between bowls along with brisket.
Ladle pho broth overtop, serve and enjoy immediately with many toppings.
See post for more detailed 2 day, instant pot, and crockpot instructions.