Pour the stock into a large, heavy pan. Add the beef, garlic, chopped coriander roots, cinnamon sticks, star anise, soy sauce, fish sauce and sugar. Bring to the boil, then reduce the heat to low and simmer for 30 minutes.
Skim off any form that rises to the surface of the liquid with a slotted spoon.
Meanwhile, divide the bean sprouts among four individual serving bowls. Remove and discard the cinnamon sticks and star anise from the stew with a slotted spoon.
Ladle the stew over the bean sprouts, garnish with the chopped scallions and chopped fresh coriander (cilantro) and serve immediately.