In a medium saucepan, heat chicken stock, tom yam paste, lime leaves, lemon grass, lime juice, fish sauce, chiles, and sugar. Once it has come to a boil, add mushrooms and simmer for 2 minutes. Add shrimp and scallions and cook for 2 to 3 minutes allowing the shrimp to cook but still be tender. Sprinkle with cilantro and serve immediately. Place extra cilantro on the table for people to add more.
Recipe courtesy of Nigella Lawson 2007