Shell and devein the prawns, reserving the shells. Rinse the shells and place them in a large pan with the chicken stock. Bruise the lemon grass stalks, and add them to the stock along with half of the lime leaves. Bring to the boil, then reduce heat to low, and simmer gently until the lemon grass changes colour, and the stock becomes fragrant, about 5 minutes. Sieve the stock and return to the pan. Discard the solids.
Return the stock to a simmer, and add the mushrooms and prawns. Cook until the prawns are pink. Stir in the fish sauce, lime juice, almost all the green onions, coriander, red chillies and remaining lime leaves. Taste, and adjust seasoning if necessary. The soup should be sour, salty, spicy and hot. Garnish with remaining onions.