Char the ginger and onion directly over an open flame or in a hot pan until blackened, then rinse under running water and set aside.
In a large pot, cover the beef bones with water and bring to a boil. Boil for 10 minutes, then drain and rinse the bones to remove impurities.
Return the cleaned bones to the pot and fill with 8 cups of fresh water. Add the charred ginger and onion, salt, sugar, fish sauce, star anise, and cloves. Bring to a boil, then reduce heat and simmer for 2 hours, skimming any scum that rises to the surface.
In the meantime, prepare the rice noodles according to package instructions. Drain and set aside.
Once the broth is ready, strain it into a clean pot, discarding the solids.
To serve, place some rice noodles into a bowl and top with sliced beef sirloin. Pour hot broth over the top, which will cook the thin slices of beef.
Garnish with cilantro, chilli peppers, scallions, and Thai basil. Serve with lime wedges on the side.