For the meat broth: Preheat the oven to 500 degrees F.
Roast the chicken bones until amber, about 1 hour. Set aside.
Sweat the onions, garlic, ginger, shiitakes and green onions in some oil in a stockpot.
Add enough water to fill the pot halfway, then add the pork bones and roasted chicken bones. (Add more water if needed to make sure everything is submerged.) Bring pot to a simmer, then simmer 24 hours.
For the dashi: Heat 1 1/4 gallons water to 140 degrees F. Add the bonito and let steep, keeping temperature constant. Strain the bonito, then add the kombu and let steep 1 hour, keeping temperature constant at 140 degrees F. Strain.
For the braised oxtail: Brown off oxtail in some oil in a roasting pan. Deglaze with 2 cups water, getting all the fond off the bottom of the pan.
Add 5 quarts meat broth, soy sauce, sugar, mirin, garlic, ginger, star anise, peppercorns and bay leaf and bring to a simmer. Simmer until meat is tender, about 2 1/2 hours.
Remove the oxtail and reserve, then strain the braising liquid and reserve.
For the oxtail saimin: Bring a pot of water to a boil and add saimin noodles, then cook for 3 minutes. Meanwhile, add 2 ounces oxtail braising liquid and 10 ounces meat broth to a bowl. When noodles are finished, add to soup. Add 8 ounces braised oxtail, followed by the mustard cabbage, egg, kamaboko, ginger, cilantro, peanuts and scallions.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.