1, MAKE THE BROTH
Place the onions and ginger directly on the cooking grate of a medium-hot charcoal or gas grill or a gas stove with a medium flame, or on a medium-hot burner of an electric stove. Let the skin burn (if you’re working indoors, turn on the exhaust fan and open a window), using tongs to rotate the onions and ginger occasionally and to grab and discard any flyaway onion skin. After 15 minutes, the onions and ginger will have softened slightly and become sweetly fragrant. There may even be some bubbling. You do not have to blacken the entire surface. When amply charred, remove from the heat and let cool.
2, Rinse the cooled onions under warm running water, rubbing off the charred skin. Trim off and discard the blackened root and stem ends. Use a vegetable peeler, paring knife, or the edge of a teaspoon to remove the ginger skin. Hold it under warm water to wash off any blackened bits. Halve the ginger lengthwise and bruise lightly with the broad side of a cleaver or chef’s knife. Set the onions and ginger aside.
3, To achieve a clear broth, you must first parboil and rinse the beef bones. Put them in a stockpot (about 12-quart capacity) and add cold water just to cover. Bring to a boil over high heat and boil vigorously for 2 to 3 minutes to release the impurities. Dump the bones and water into the sink (make sure it is clean), and then rinse the bones with water to wash off any clinging residue. Quickly scrub the stockpot clean and return the bones to the pot.
4, Pour in the 6 quarts water, bring to a boil over high heat, and lower the heat to a gentle simmer. Use a ladle or large, shallow spoon to skim off any scum that rises to the top. Add the onions, ginger, star anise, cloves, cinnamon stick, beef, salt, fish sauce, and rock sugar and cook, uncovered, for 1½ hours, adjusting the heat if needed to maintain a simmer.
5, At this point, the boneless meat should be slightly chewy but not tough. Press it and it should feel like the flesh at the base of your thumb. When it is cooked to your liking, use tongs to transfer it to a bowl of cold water to cover. Let the meat soak for 10 minutes to prevent it from drying out and turning dark. Drain the meat, set aside on a plate to cool completely, then cover with plastic wrap and refrigerate. Meanwhile, maintain the broth at a steady simmer for 1½ hours longer.
6, Strain the broth through a fine-mesh sieve (or a coarse-mesh sieve lined with cheesecloth) positioned over a pot. If desired, remove any bits of gelatinous tendon from the bones to add to the cooked beef in the refrigerator. Discard the remaining solids. Use a ladle to skim as much fat from the top of the broth as you like. (To make this task easier, you can cool the broth, refrigerate overnight, lift off the solidified fat, and then reheat before continuing.) Taste and adjust the flavor with salt, fish sauce, and rock sugar. There should be about 4 quarts (16 cups) broth.
7, ASSEMBLE THE BOWLS
If using dried noodles, cover them with hot tap water and let soak for 15 to 20 minutes, or until they are pliable and opaque. Drain in a colander. If using fresh rice noodles, untangle them, place in a colander, and rinse briefly under cold running water.
8, Cut the cooked beef across the grain into slices about 1/16 inch thick. For the best results, make sure it is cold. Freeze the raw beef for 15 minutes, then slice it across the grain into pieces 1/16 inch thick. Set all the beef slices aside. Ready the yellow onion, scallions, cilantro, and pepper for adding to the bowls. Arrange the garnishes on a plate and put on the table.
9, To ensure good timing, bring the broth to a simmer over medium heat as you are assembling the bowls. (For an extra treat, drop in any unused white scallion sections and let them poach in the broth. Add the poached scallion sections—called hành chần—to a few lucky bowls when ladling out the broth.)
At the same time, fill a large pot with water and bring to a rolling boil. For each bowl, place a portion of the noodles on a vertical-handle strainer (or mesh sieve) and dunk the noodles in the boiling water. As soon as they have collapsed and lost their stiffness (10 to 20 seconds), pull the strainer from the water, letting the water drain back into the pot. Empty the noodles into a bowl. If you like, once you have finished blanching the noodles, you can blanch the bean sprouts for 30 seconds. They should wilt slightly but retain some crunch. Drain and add to the garnishes.
10, Top each bowl of noodles with cooked and raw beef, arranging the slices flat. Place a mound of yellow onion in the center and shower some scallion and cilantro on top. Finish with a sprinkle of pepper.
11, Raise the heat and bring the broth to a rolling boil. Do a final tasting and make any last-minute flavor adjustments. Ladle about 2 cups broth into each bowl, distributing the hot liquid evenly to warm all the ingredients. Serve immediately with the plate of garnishes.