Hot-and-Sour Prawn Soup with Lemon Grass

Shell and devein the prawns, reserving the shells. Rinse the shells and place them in a large saucepan with the chicken stock. Bruise the lemon grass stalks, and add them to the broth along with half of the lime leaves. Bring to a boil, then reduce heat to low, and simmer gently until the lemon grass changes color, and the stock becomes fragrant, about 5 minutes. Strain the stock and return to the saucepan. 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, 2 tablespoons green onion, cilantro, red chilies, and remaining lime leaves. Taste, and adjust seasoning if necessary. The soup should be sour, salty, spicy and hot. Garnish with remaining green onions.