Peel and devein prawns, reserving the shells. Heat 6 cups (11/2 L) of water in a large saucepan. Add prawn shells, lemon grass, lime leaves, ginger and tamarind. Bring to a boil, then strain into a large bowl. Return stock to pan and bring back to a boil. Add pumpkin, reduce heat to low and simmer for six minutes.
Add prawns. As they turn pink, add mushrooms, tomatoes, green onions, lime juice, chili and fish sauce (if using). Stir. Season to taste. Garnish with coriander and mint leaves.
Recipe Notes
Per serving: 98 calories, 7 g protein, 1 g fat (0 g saturated fat), 18 g carbohydrates, 4 g fibre, 32 mg cholesterol, 49 mg sodium