1. Wash the unpeeled shrimp well. Peel and devain them, keeping the peelings. Wash the shrimp again and pat them dry. 2. Cut 3 X 2" pieces from each stick of lemon grass starting from the bottom end, and crush lightly. Discard the top. 3. Combine the stock, lemon grass, lime leaves and shrimp peelings in a pan. Bring to a boil and simmer gently for 20 minutes. Strain. Add fish sauce, lime juice and chile paste. Adjust seasoning if required (Fish sauce and lime juice). 4. Add the drained mushrooms. The soup can be stored like this. 5. Before serving cut the chilies into very fine rounds. Wash and dry the coriander leaves. 6. Bring soup to a boil. Add shrimp and cook 2 minutes or until the shrimp are opaque. Add garnishes to serving bowl(s).