Roast marrow bones at 400 degrees until they brown, about 15-20 minutes. Remove bones then roast onions until blackened, about 10 minutes.
Fill a stock pot with one gallon of water and place over medium heat. Add bones, ginger, star anise, cinnamon, fish sauce, and onions. Bring to a simmer for 5-10 hours. Adjust heat to retain a light simmer. Taste periodically and season with salt and pepper. Add more water if you want to dilute the flavor. Keep hot until ready to serve.
Cook the rice noodles according to the package instructions.
Take a deep soup bowl and fill one-portion’s worth of noodles. Add several slices of thinly cut steak. Fill bowl with hot broth. Top with your choice of seasonings including cilantro, scallions, salt and pepper. Garnish with basil, bean sprouts, sriracha sauce, hoisin sauce, or lime juice, as desired.