1. Preheat oven to 400°F (200°C). Place halved onions and sliced ginger on a baking sheet and char in the oven for 20-25 minutes until softened and slightly blackened. Alternatively, char directly over a gas flame. Set aside.

2. Meanwhile, toast star anise, cinnamon sticks, cloves, black cardamom pod, and coriander seeds in a dry pan over medium heat for 2-3 minutes until fragrant. Place them in a spice bag or cheesecloth.

3. Blanch beef bones: Place beef marrow bones in a large pot, cover with cold water, and bring to a rolling boil. Boil for 10-15 minutes, then drain and rinse the bones thoroughly under cold water to remove impurities. Rinse the pot as well.

4. Return the blanched bones and beef chuck/brisket to the clean pot. Add the charred onions and ginger, the spice bag, fish sauce, sugar, and salt. Cover with about 6-8 liters of fresh cold water.

5. Bring the pot to a boil, then immediately reduce heat to a very gentle simmer. Skim off any foam or impurities that rise to the surface. Simmer for at least 3-4 hours, or up to 6 hours, keeping the lid slightly ajar. For even richer flavor, simmer the bones for 8-10 hours, removing the beef chuck/brisket after 2-3 hours to prevent it from becoming too tough. Taste and adjust seasoning as needed.

6. Carefully remove the cooked beef chuck/brisket from the broth and let it cool before thinly slicing against the grain. Strain the broth through a fine-mesh sieve into a clean pot, discarding the bones, aromatics, and spices. Reheat the broth gently.

7. Prepare rice noodles according to package instructions. Usually, this involves briefly cooking fresh noodles in boiling water or soaking dried noodles in hot water until tender.

8. To assemble: Divide cooked noodles among serving bowls. Arrange thinly sliced beef (cooked chuck/brisket, or very thinly sliced raw beef which will cook in the hot broth) over the noodles. Ladle the hot, simmering broth over the beef and noodles.

9. Serve immediately with a plate of fresh garnishes: bean sprouts, Thai basil, cilantro, lime wedges, and sliced jalapeños. Offer hoisin sauce and Sriracha on the side for diners to customize their bowls.