Preheat the oven to 425 degrees F (220 degrees C).
Place beef bones on a baking sheet and roast in the preheated oven until browned, about 1 hour.
Place onion halves on a second baking sheet and roast on another rack until blackened and soft, about 45 minutes.
Transfer beef bones and onion halves to a large stockpot. Add ginger, salt, star anise, fish sauce, and 4 quarts water; bring to a boil. Reduce heat to low and simmer for 6 to 10 hours. Strain the broth into a saucepan and set aside.
Place rice noodles in a large bowl filled with room temperature water. Let soak for 1 hour. Drain.
When noodles have soaked for 1 hour, heat up the reserved broth by bringing it to a simmer.
Bring a large pot of water to a boil. Cook the noodles in the boiling water for 1 minute. Drain.
Divide noodles among 4 serving bowls; top with sirloin, cilantro, and green onion. Ladle hot broth over the top. Stir and let sit until beef is partially cooked and no longer pink, 1 to 2 minutes.
Serve with bean sprouts, Thai basil, lime wedges, hoisin sauce, and chile-garlic sauce on the side.