Thai Soup with Prawns

Rinse, remove heads, shells and intestines from prawns with a sharp knife (devein). Rinse and drain shells and heads in a colander. In a separate colander, rinse and drain shelled prawns. 

Heat oil in a wok and lightly stir-fry prawn shells and heads. Add 250 ml (approximately 1 cup) broth. Cover and simmer over medium heat, about 20 minutes. Strain through a fine sieve into another container.

Peel garlic, carrots, ginger and galangal. Cut carrots into pieces, slice ginger and galangal, and finely chop garlic. Rinse and thinly slice lemongrass. Rinse chile and thinly slice crosswise into rings. Drain water chestnuts.

Bring strained cooking liquid and remaining broth to a boil. Stir in curry paste with some of the tamarind paste. Add garlic, carrots, ginger, galangal, lemongrass, chile, kaffir lime leaves and water chestnuts. Simmer over low heat, about 5 minutes. Add prawns and simmer over low heat until cooked through, about 5 minutes (do not let boil).