Thai Hot and Sour Shrimp Soup

Name: Thai Hot and Sour Shrimp Soup
Ingredients: 1 pound medium shrimp, shelled and deveined, butterflied, shells reserved; 1/4 cup lime juice; 4 teaspoons Thai fish sauce; 1/4 cup minced fresh green chilies; 2 tablespoons very finely-minced lemongrass, from tender inner part of stalk, plus 2 tablespoons thinly-sliced; 2 teaspoons vegetable oil; 4 cups chicken stock; 2 tablespoons crumbled dried Kefir lime leaves (available in Asian markets or specialty food stores); 2 tablespoons finely-minced fresh coriander roots and stems; 2 thick slices galanga root (available in Asian markets or specialty food stores); 2 medium shallots, coarsely chopped; 2 teaspoons dried tamarind pulp (available in Asian markets or specialty food stores); 1/2 cup straw mushrooms, rinsed; 2 heaping tablespoons fresh coriander leaves; 2 teaspoons minced scallion