Bring a pot of water to the boil,turn off the heat and soak the vermicelli noodles for a few minutes until soft. Rinse under cold running water until cool and drain.
Cut off the coriander roots from the stems. Cut a length of stems (from the bottom) about 2 inches long. Reserve 1/3 cup of the leaves for later.
In a mortar and pestle,pound the coriander root and the 2 inches of stem with the garlic,chilli,peppercorns and onion until they form a paste.
Soak the tamarind in 1 cup of warm water for several minutes until softened (if not using pulp). Squeeze and work the pulp with your hands to loosen the fibres. Soak for a further 30 minutes then strain,pushing the pulp against the strainer to get the tamarind water. Discard the pulp.
Heat a wok over high heat and add the peanut oil,then the coriander paste. Stir-fry for a few minutes until softened,stirring all the time.
Now add the pork mince and lime leaves and stir-fry a few more minutes until the pork is cooked through. Add the palm sugar,fish sauce and the tamarind water. Bring to the boil,stirring all the while,and taste. It should taste sweet,salty and sour all at once. If it needs more salt add a little fish sauce or if too rich,just add a little water.
Add the noodles,mixing them well into the pork sauce,and toss in the cucumber,Thai basil and coriander leaves.
To serve: Divide into individual bowls and serve immediately.
*Jars of ready-made tamarind puree can be bought at Asian grocers. Although the jars are convenient,I prefer the flavour of a freshly made version.