How to Make It
In a large, deep skillet, heat 1 tablespoon of the oil. Add the shiitake and season with salt and pepper. Cover and cook over moderate heat, stirring a few times, until tender, 5 minutes. Uncover and cook, stirring, until the shiitake are golden, 3 minutes. Transfer to a plate.
Add the remaining 1 tablespoon of oil to the skillet. Add the chiles, lemongrass, shrimp shells, onion and ginger and cook over moderate heat, stirring occasionally, until the onion is softened and golden, about 7 minutes. Add the fish sauce and stock and bring to a boil. Simmer over moderate heat for 10 minutes. Strain the shrimp broth into a large bowl.
Return the broth to the skillet; bring to a simmer over moderate heat. Add the shrimp and shiitake and cook until the shrimp are pink and curled, 1 minute. Add the lime juice and season with salt and pepper. Ladle the soup into bowls, garnish with the basil leaves and serve.