Preheat oven to 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 (and melted bone marrow), onion, ginger, salt, sugar, star anise, garlic, cinnamon, nutmeg, cardamom seeds and fish sauce in a large stockpot and cover with 4 liters of water. Bring to a boil and reduce heat to low. Simmer on low for 8 to 12 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.