Preheat 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 on a baking sheet and roast in the preheated oven until blackened and soft, about 45 minutes.
Place bones, onion, ginger, salt, star anise, and fish sauce in a large stockpot and cover with 4 quarts of water. Bring to a boil and reduce heat to low. Simmer on low for 6 to 10 hours. Strain the broth into a saucepan and set aside.
Place rice noodles in large bowl filled with room temperature water and allow to soak for 1 hour. Bring a large pot of water to a boil and after the noodles have soaked, place them in the boiling water for 1 minute. Bring stock to a simmer.
Divide noodles among 4 serving bowls; top with sirloin, cilantro, and green onion. Pour hot broth over the top. Stir and let sit until the 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.
For best results, use shank and knee bones.
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