For the most authentic Vietnamese pho, making the flavorful, fragrant broth from scratch is a must. Blanching the bones and meat first, then rinsing them well, helps create a beautifully clear, rich broth. Rice vermicelli comes in several widths -- we prefer the thin variety but you can use medium or broad.
Put the beef bones, oxtail, beef chuck and 1 tablespoon salt in a large pot and add cold water to cover by 2 inches. Bring to a boil over high heat and boil vigorously until the surface of the water is foamy, about 10 minutes. Drain the bones and meat in a colander (discarding the liquid) and rinse well with cold water. Clean the inside of the pot, put the blanched bones and meat back into it and add enough cold water to cover by 2 inches. Bring to a boil over high heat then reduce to medium-low and simmer for 15 minutes, skimming and discarding any foam that rises to the surface.
Meanwhile char the onion and ginger over an open flame until well blackened on both sides. (Alternatively you may place them on a foil-lined baking sheet and broil on high, turning once, until well charred, about 10 minutes.) Add the onion, ginger, fish sauce, cloves, star anise, cinnamon, black peppercorns and rock sugar to the pot and continue to simmer. Skim the surface of the soup often to remove the fat and foam that rises. If necessary, add water during cooking to keep the bones submerged. Cook until a paring knife pierces the chuck easily and comes out with little resistance and the oxtail meat pulls easily away from the bone, 3 1/2 to 4 hours.
Remove the meat and bones from the pot with a slotted spoon or spider to a rimmed baking sheet. Discard the larger bones and let the chuck and oxtail cool enough to handle, 10 minutes. Meanwhile pour the broth through a fine-mesh strainer into a clean pot and keep hot over medium-low heat. (You should have about 12 cups of broth.) Put the rice noodles into a large bowl and cover with hot water. Let the noodles soak until soft, 10 to 15 minutes, then drain and divide between 6 serving bowls.
Pick the oxtail meat from the bones and divide it between the bowls. Thinly slice the beef chuck across the grain of the meat and divide it between the bowls. Divide the sliced eye of round between the bowls and then ladle the hot broth over. Top with the cilantro and onion and serve with the bean sprouts, basil, lime, sriracha and hoisin on the side.
You can find yellow rock sugar in Asian markets and online. If none is available, you may substitute granulated sugar.
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