For the sesame seed paste: In a mortar and pestle, crush the sesame seeds. Add the sugar, soy, cilantro, sesame oil and chiles. Mash together until a thin paste forms. Transfer the mixture to a small bowl and stir in the lime juice.
For the spicy fish sauce: In a small bowl, combine the fish sauce, sugar, garlic, chiles and lime juice and stir well to dissolve the sugar.
For the spice sachet: Put the cardamom, star anise, cassia bark and licorice root on a small piece of cheesecloth and tie with some kitchen twine.
For the herb sachet: Put the cilantro, ginger, lemongrass, basil and garlic on a small piece of cheesecloth and tie with some kitchen twine.
For the soup and add-ins: Put the hot beef pho broth in a hot pot heated with a sterno or an electric fondue pot and add the spice and herb sachets. Let the mixture cook and steep for 15 minutes.
Serve the soup with platters of the "add ins"--sliced beef, spinach, mushrooms, scallions, cabbage, red onions, cilantro, basil, mint and bean sprouts--and bowls of the sesame seed paste, spicy fish sauce and chile oil for dipping. Ladle into bowls, allow the guests to choose the "add ins" they prefer and enjoy with the dipping sauces.
Yu choi is a Chinese green that can be found at specialty Asian markets. Cassia bark, black cardamom and licorice root can be found online or at specialty markets.