How to Make It
Cut the meat off the shank bones. In a medium saucepan, cover the bones with water and bring to a boil. Prepare a medium bowl of ice water. Transfer the bones to the ice water to cool, then pat dry.
Heat 1 tablespoon of the oil in a large skillet. Add the onions, cut side down, and cook over moderately high heat until blackened on the bottom, about 12 minutes; remove to a plate. Heat the remaining 1 tablespoon of oil in the skillet. Spread the ginger slices in a single layer and cook over moderately high heat until blackened, about 12 minutes.
In a medium skillet, toast the star anise pods, cinnamon sticks, cloves and crushed red pepper over moderate heat for 2 minutes, shaking the skillet.
In a large stockpot, cover the shank meat and oxtails with the water and bring to a boil. Skim, then add the lemongrass, garlic, blackened onions and ginger and the toasted spices and simmer over moderately low heat, skimming occasionally, until the meats are tender, about 2 hours. Strain the broth; refrigerate the broth and the meats separately overnight or for up to 2 days.
Discard the fat from the broth. Bring the broth to a boil and keep hot. Thinly slice the meats, discarding any bones.
In a large bowl, soak the rice noodles in water until they are pliable, about 30 minutes. Bring a large saucepan of water to a boil.
In a medium saucepan of boiling salted water, blanch the bok choy, asparagus and snow peas separately until crisp-tender; remove the vegetables to a platter. Add the shank bones to the saucepan and cook over moderate heat for 2 minutes. Drain and let cool slightly, then use a small spoon to scoop out the marrow.
Preheat a grill pan. Season the sirloin with salt and pepper and cook over high heat until rare, about 2 minutes per side. Transfer to a carving board and let rest for 5 minutes. Thinly slice the sirloin.
Drain the rice noodles and add a handful to a long-handled strainer that will fit into the saucepan of boiling water. Lower it into the water and cook the noodles until barely tender, 30 seconds to 1 minute. Drain the noodles well and transfer them to a large soup bowl. Add some of the shank and oxtail meats and cover with hot broth. Add a piece of marrow and slices of sirloin and serve, allowing the diner to garnish the soup as desired. Repeat for the remaining diners.