Galangal, a root similar to ginger, kaffir lime leaves, lemongrass, and fish sauce give this soup authentic flavor. Substitute ginger if you can't find galangal, and use 3 (2-inch) lime rind strips in place of the leaves. Reserve the shrimp shells; you'll need them to make the broth. Serve with rice noodles, if desired.
1 1/2 pounds medium shrimp
9 1/2 cups water, divided
1/2 cup chopped peeled fresh galangal (about 2 ounces)
1/2 cup (2-inch) pieces peeled fresh lemongrass (about 4 stalks)
6 fresh or frozen kaffir lime leaves
1/2 cup canned straw mushrooms, quartered
2 tablespoons roasted red chili paste (such as Thai Kitchen)
Peel and devein shrimp, reserving shells. Combine shrimp shells and 6 cups water in a Dutch oven; bring to a simmer. Cook 1 hour. Strain broth through a sieve into a bowl; discard solids. Combine broth and remaining 3 1/2 cups water in a large saucepan; bring to a boil. Add galangal, lemongrass, and kaffir lime leaves to pan; simmer 10 minutes. Strain broth mixture through a sieve into a bowl; discard solids. Return broth mixture to pan. Add mushrooms, chili paste, fish sauce, and chiles; bring to a boil. Stir in shrimp, green onions, and cilantro; cook 3 minutes or until shrimp are done. Discard chiles. Stir in fresh lime juice. Ladle 2 cups soup into each of 4 bowls; sprinkle evenly with peanuts. Serve with lime wedges.