To make the stock, place the beef knuckle in a very large stock pot. Season with salt, and fill pot with 7 1/2 litres of water. Bring to a boil and cook for about 2 hours.
Skim fat from the surface of the stock, and add the oxtail, radish and onions. Tie the anise pods, cinnamon stick, cloves, peppercorns and ginger in a cheesecloth or similar; add to the soup. Stir in sugar, salt and fish sauce. Simmer over medium-low heat for at least 4 more hours (the longer, the better). At the end of cooking, taste and add salt as needed. Strain stock and return to the pot to keep at a simmer. Discard spices and bones.
To make the soup, bring a large pot of lightly salted water to a boil. Soak the rice noodles in water for about 20 minutes, then cook in boiling water until soft, but not mushy, just a couple of minutes. Slice the frozen beef paper thin. The meat must be thin enough to cook instantly.
Place some noodles into each bowl, and top with a few raw beef slices. Ladle boiling broth over the beef and noodles in the bowl. Serve with hoisin sauce and chilli sauce and with a plate of bean sprouts, basil, slices of fresh chilli and lime out at the table for everyone to add their own toppings.