Pound the garlic, shallots, coriander roots, mint leaves, ginger, galangal, lemon grass, chillies and kaffir lime leaves.
Remove the shells & heads from the prawns, then the intestinal tracts.
Butterfly prawns open and set aside
Heat oil in a wide saucepan and fry prawn heads and shells over moderate heat until they are pinkish in colour, remove from the pan.
In the same pan, fry aromatics and herbs until fragrant and just beginning to colour.
Return prawn heads and shells to the pan and stir to incorporate.
Add stock and bring to the simmer over gentle heat then cook for 45 minutes
Strain through a fine meshed sieve, getting rid of the solids
Return soup to rinsed out pan, stir in the fish sauce and lime juice whilst the soup is very hot, taste and adjust seasoning if necessary.
Simmer prawns & mushrooms in soup for 2 minutes and transfer soup and prawns to hot serving bowls
Add tomato slices, Vietnamese mint leaves, lime leaves and chilli.