For the hot pot broth: Rinse the chickens really well before making the broth. Make sure you rinse out the cavities as well as the outsides to remove as much blood as possible.
In a large pot, combine the chickens and 16 cups ice water. Bring to a boil and then immediately reduce the heat to a low simmer. Simmer the broth for 3 hours.
Trim the lemongrass stalk, then bruise it by hitting it with the back of a chef's knife, down the length of the stalk. Add to the broth along with the onions, tamarind paste, chile paste, palm sugar, garlic and ginger and simmer for an additional hour.
Season the broth with salt and pepper. Add the lime juice and additional chile paste if you want a spicier broth. Strain out the solids and reserve the chicken meat for another use.
For the braised oxtail: Meanwhile, preheat the oven to 350 degrees F. Sprinkle the oxtail with the salt and pepper and dredge in the flour.
Heat a wok over high heat and add the oil. Working in batches, sear the oxtail pieces until browned. Transfer to an oven-safe casserole dish and fill the casserole dish with enough of the beef broth to cover the oxtail pieces three-quarters of the way. Cover with aluminum foil and place in the oven. Cook until the oxtail is tender, 3 to 4 hours. Allow the oxtail to cool in the braising liquid. Keep the oven temperature at 350 degrees F.
For the Vietnamese meatballs: In a large bowl, mix together the beef, pork, onions, scallions, garlic, ginger, lemongrass, soy sauce, salt and pepper. Form 2- to 3-ounce meatballs from the mixture and arrange them on a rimmed baking sheet. Bake the meatballs until fully cooked through, 30 to 45 minutes.
For the noodles: Bring 16 cups water to a boil and add the egg noodles. Cook until tender, then drain and rinse in cold water.
For plating, arrange the tofu, cabbage, carrots, daikon, watercress, Thai basil and egg noodles around the rim of a large hot pot bowl. Quarter 5 Vietnamese meatballs and add to the rim; add 4 to 6 pieces of oxtail to the rim. Fill the center of the bowl with the hot pot broth and serve family-style over a portable burner.