To prepare the garlic oil - Chop a head of garlic - If you wish the garlic to be crispy, soak in water, then thoroughly dry with kitchen paper towel. Put the chopped dry garlic in a small sauce pan, barely cover with oil. Cook on low heat until gentle bubbles form and the garlic turns golden. Be careful to not let the garlic burn. Strain the garlic from the oil. If you don't use all, it can be kept in an airtight container or jar in the refrigerator for a few weeks. Put the garlic oil in a jar or closed container when cool.
To make the chili vinegar, slice the chili, add with the garlic and a pinch of salt in a mortar and pestle, and pound til mashed, add vinegar, pour into a small bowl. A simpler version is to simply slice the chilis and mince the garlic and put in a small bowl, and add some vinegar.
Wash the duck whole legs (if using half a duck, cut into 4 pieces) and dry.
To make the broth - Turn Instant Pot Saute function. Add the spices (anise, cinnamon, goji berries if used, galangal slices, szechuan peppercorn, coriander seeds) and stir fry till fragrant. Add a little oil, and the minced garlic, stir fry for 1-2 minutes. Add the chicken broth to the Instant Pot and the seasoning (soy sauces and oyster sauce) and sugar.
Add the duck pieces to the pot. Add water to cover. Close and lock the lid, turn the knob to lock. Set on Pressure Cook High 18 minutes with natural release.
Wash and dry the vegetables and garnishes.
In the meantime, boil water and cook the noodles according to the package instruction. It's best to divide the noodles into 4 portions. Put a noodles portion in a slotted laddle, dip into the boiling water and when almost cooked, add a handful of beansprouts and blanch for 5 to 10 seconds (the beansprouts should still be crunchy), and put in the indidivual serving bowl. Continue with the other portions of noodles and bean sprouts.
After the steam is released, open the lid. Remove the duck legs and put a duck leg on top of the noodles and bean sprouts. If using the half duck, remove and set aside. When cool, chop the duck into bite size pieces (you can remove the bones if you wish) and put them on top of the noodles and bean sprouts.
Set the Instant Pot to Saute. Taste and adjust seasoning if necessary. Skim the fat rising on top. Add the pok choy or green lettuce leaves in the hot broth to blanch them for 10 seconds, and put the vegetables on top of the bowls of noodles and bean sprouts.
Laddle the broth into the bowls of noodles, drizzle with the garlic oil, fried garlic, cilantro and Thai celery leaves if used. Serve with the chili vinegar on the side.