Shell and devein the prawns if you need to. Reserve the shells.
Heat oil over medium heat in a pan, add prawn shells, garlic and chilies, cook stirring (about 5 minutes or until the shells are red and the mixture is fragrant).
Add lemongrass, lime leaves, water, juice, sugar, sauce and coriander roots and simmer covered (for 30 minutes).Strain the stock.
Place the strained stock, prawns, coriander leaves and shallots into a clean pan, and simmer covered (for about 3 minutes until the prawns are tender).