In a large pot, submerge the oxtail in cold water, add 3 tablespoons salt and soak for 1 hour, then drain.
To make the spice pouch, dry roast each ingredient separately in a frying pan over medium heat until fragrant.
Cool, then coarsely grind the spice using a mortar and pestle or small spice grinder. Add the ground spices to the muslin square and tie up tightly in a knot. Set aside.
Heat a barbecue grill or chargill pan over medium-high heat and grill the unpeeled garlic, onions and ginger evenly for 15 minutes in total until all sides are blackened.
Now peel the blackened skins and discard them, and then roughly chop. By doing this, the garlic, onion and ginger becomes sweet and fragrant, releasing more flavour into the stock.
Put the oxtail, brisket and 200 fluid ounces/ 6 litres cold water in a stockpot and bring to the boil. While the stock is boiling, constantly skim any impurities off the surface for 15 minutes (this will ensure a clean, clear broth), then reduce the heat to a low simmer.
Add the fish sauce, remaining 1 tablespoon salt, rock sugar, garlic, onions, ginger, and spice pouch.
Cover and simmer for 3 hours, or until the stock has reduced to almost 1/2.
Strain the stock through a muslin cloth. Remove the brisket, set aside to cool, then thinly slice.
Blanch each portion of noodles in boiling water for 20 seconds.
Drain, then transfer to a serving bowl.
Place 3 or 4 slices of brisket on top of the noodles, followed by 3 or 4 pieces of raw sirloin. Pour over the hot stock to cover the noodles and beef.
Serve with 1 tablespoon spring onion, a pinch of black pepper, and a coriander sprig to each bowl.