For the broth: Place a large colander in the sink. Place the oxtails, marrow bones, beef shin and pork hocks in a large (at least 8-quart) pot. Cover with cold water and bring to a boil over high heat. Let boil for a few minutes, then remove from the heat. Scoop out the bones and meat into the colander and discard the water. Wash the pot. Rinse off the bones and meat and put them back into the pot. Fill with fresh cold water to cover by 1 inch. Add the yellow onion halves and 1 tablespoon salt. Bring to a boil and reduce to a simmer. Periodically skim the foam and some, but not all, of the fat. Simmer until the meats are tender but not falling apart, 1 to 1 1/2 hours, then remove the beef shins and pork hocks. (Do not remove the oxtails or pork neck bones at this stage, if using.) When the meats are completely cool, wrap them in plastic wrap and refrigerate until ready to serve.
Meanwhile, cut the tops off of the lemongrass stalks, about 6 inches from the base, and discard. Smash 6 of the bottom stalks with a meat pounder to bruise and open up the lemongrass. For the remaining 4 stalks, peel off 2 or 3 layers until you reach the smoother, more tender core. Quarter the peeled stalks lengthwise and then slice thinly across. Reserve the sliced lemongrass for the satay.
Add the bruised lemongrass stalks to the broth and continue to simmer until the broth has reduced by about 10 percent from its original amount, about 1 more hour, then strain the broth into a clean pot. If using, pick the meat from the cooled oxtail and pork neck bones (save the beef shins and pork hocks for later) and reserve; discard the bones, onion and lemongrass.
Bring the shrimp paste and 2 cups water to a simmer in a small saucepan, then let simmer for 5 minutes. Skim off the foam. Remove from the heat and let sit for 15 minutes for the fine solids to settle to the bottom. Pour the shrimp water into the large pot of stock, leaving the fine solids behind.
For the satay: Heat the vegetable oil in a medium skillet over medium heat. Add the annatto seeds and let the seeds infuse the oil and turn it red, about 1 minute. Scoop out the seeds with a spoon and discard. Add the shallots to the annatto oil and cook until starting to brown, about 2 minutes. Add the garlic, chile flakes and the reserved chopped lemongrass and cook until the garlic starts to brown, about 3 minutes. Add 2 tablespoons fish sauce, 4 teaspoons sugar and 1/2 teaspoon salt. Stir to incorporate and then cook, stirring occasionally, until the mixture is sticky and thickened slightly to the texture of loose jam. (The satay is done when a spoon dragged across the bottom of the skillet leaves a trail and the oil begins to separate from the solids.) Turn off the heat and remove half of the satay for serving. Add about 1 cup of the broth to the remaining satay in the skillet and bring to a simmer. Simmer for 5 minutes to infuse, then strain back into the pot with the broth.
Add the remaining tablespoon fish sauce, teaspoon sugar and 2 teaspoons salt to the broth. Simmer for 15 minutes for the flavors to meld. Add the pork hocks back into the simmering broth to re-warm, and slice the beef shin meat . Before serving, taste the broth and add more seasoning, if necessary.
Meanwhile, bring a large pot of water to a boil and cook the noodles according to the manufacturer's instructions. Drain very well, and divide among 8 bowls. Top each bowl with a pork hock, if using, some slices of shin, some oxtail and pork neck meat, if using; top with scallions, white onion and cilantro. Pour hot broth into the bowls to completely cover the noodles.
Serve with the reserved satay and suggested garnishes on the side.
The rice noodles for this dish can be found in Chinese and Southeast Asian markets. They are similar to spaghetti in shape and thickness. Often "Bun Bo Hue" will be printed on the noodle package.
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