Clean the beef bones under running water and add them to a pot. Cut the ginger and onion in half. Place a skillet over high heat. Add the onion and ginger cut side down. Cook for a few minutes until it's charred, then turn. Remove and add to the pot with the bones. Add beef stock, water, coriander, cinnamon, cardamom, star anise, fish sauce, peppercorns, and brown sugar. Place over medium heat. Bring to a boil, then simmer for 30 - 40 minutes. Strain broth into another pot and discard bones and spices.
rice noodles and meat
Clean the rice noodles according to the package instructions. Cut the steak into thin stripes.
Divide the rice noodles between four plates. Add the thinly slices beef steak. Ladle the soup over the steak; this will cook it to medium rare. Arrange the sliced onion, sprouts, basil, and coriander on top. Optionally, add some chili for spiciness.