Rinse the beef and bones thoroughly, place them in a pot, add salt, and bring to a boil. Skim off any foam as it forms. Lightly crush the lemongrass and add it to the pot. Once the broth becomes clear, reduce the heat and simmer until the beef is cooked through. Remove the beef, let it cool, wrap it tightly, and refrigerate to make it easier to slice thinly. Continue simmering the bones for another 1–2 hours. Occasionally add more water to keep about 8 quarts (about 8 liters) in the pot.

Sauté the minced garlic, lemongrass, and dried chili flakes in olive oil until golden. For a richer red color, you can add annatto oil if desired. Slice the beef thinly and set aside. Chop the green onions and Vietnamese coriander. Arrange the noodles in bowls, top with beef, green onions, and Vietnamese coriander. Add a spoonful of the sautéed lemongrass and chili mixture (or stir it into the broth if you prefer). Bring the broth to a boil and ladle it over the bowls to serve.