In a large pot, add beef bones and enough water to cover. Bring to a boil and let it boil for 5 minutes. Drain the bones and rinse them with cold water to remove any impurities.
In the same pot, add the bones, onion, ginger, cinnamon sticks, star anise, cloves, coriander seeds, fennel seeds, salt, sugar, and 6 quarts of water. Bring to a boil and reduce heat to low. Simmer for 6-8 hours, skimming any foam or impurities that rise to the top.
Strain the broth through a fine mesh strainer into another pot. Discard the bones and spices.
Bring the broth to a boil and add the beef sirloin. Cook for 2-3 minutes or until the beef is no longer pink. Remove the beef from the pot and set aside.
Cook the rice noodles according to the package instructions. Drain and rinse with cold water.
Divide the noodles among 4 large bowls. Top with the cooked beef, green onions, bean sprouts, cilantro, and mint.
Ladle the hot broth over the ingredients in each bowl.
Serve with fish sauce, lime wedges, and additional herbs for garnish.