Preheat the oven to 400°F (200°C). Place the marrow bones on a baking sheet and roast them until richly browned, about 15 to 20 minutes. Remove the bones, then roast the halved onion until the cut sides are deeply charred, around 10 minutes more.

In a large stockpot, pour in one gallon of water and set over medium heat. Add the roasted bones and onions, followed by the ginger slices, star anise, cinnamon stick, and fish sauce. Bring the mixture to a gentle boil, then reduce the heat and let it simmer uncovered for 5 to 10 hours. Skim off any foam or impurities that rise to the surface during the cooking process. Taste occasionally and season with salt and pepper to your preference. If the broth becomes too concentrated, add more water to balance the flavor. Keep the broth hot until it's time to serve.

Prepare the rice noodles according to the directions on the package. Drain and rinse under cool water to prevent sticking.

To assemble, place a serving of noodles into a deep soup bowl. Arrange a few slices of raw beef over the noodles. Pour the steaming hot broth directly over the meat, allowing the heat to cook it instantly.

Garnish as desired with your choice of toppings—herbs like cilantro and basil, crunchy bean sprouts, spicy jalapeño, or a squeeze of lime. Add hoisin or sriracha for extra flavor, and finish with a sprinkle of salt and black pepper if needed. Serve piping hot.