Place the beef bones, water, garlic, ginger, celery, spring onions, star anise, cinnamon stick, sugar, white pepper, and salt in a large saucepan. Bring the mixture to a boil over medium-high heat, then reduce to a simmer. Cook gently for 3 hours, skimming off any foam or impurities that rise to the surface to keep the broth clear. Once the broth is done, strain it into a clean pot, ensuring you have at least 1 liter. Add water if necessary to meet this volume. Set aside.

In a food processor, combine the minced beef, cornflour, garlic, coriander, salt, white pepper, and baking powder. Blend until the mixture is smooth and evenly combined. Transfer the mixture to a bowl, cover with cling film, and refrigerate if not cooking immediately.

When ready to cook, bring the prepared broth or a pot of water to a gentle simmer. Wet your hands to prevent sticking, then scoop a small portion of the meat mixture into your hand. Squeeze it through the gap between your thumb and forefinger to shape into a ball. Use a teaspoon to scoop the ball off your hand and drop it gently into the simmering liquid. Repeat until all the mixture is used.

Cook the meatballs for about 3 minutes, or until they float to the surface, indicating they are fully cooked. Avoid a rolling boil to prevent the meatballs from falling apart. Remove them with a slotted spoon and set aside.

Cook your chosen noodles according to the package instructions. In the last 30 seconds of cooking, add the beansprouts to soften them slightly. Divide the noodles and beansprouts among four serving bowls.

Place the cooked meatballs over the noodles, add fresh greens (blanched or raw, depending on preference), and ladle hot broth over the top. Garnish with crispy shallots, sliced chilies in soy sauce, and sambal if desired. Serve immediately.