- Heat oil over medium heat in a heavy pot. Add shrimp shells and sauté for 1 minute. Add kaffir lime leaves, galangal slices, lemongrass paste and chicken stock. Bring to a boil. Reduce heat to medium-low and simmer for 15 minutes, until broth is fragrant.
- Strain broth through a colander placed over a large bowl and return broth to pot. Discard aromatics. Stir lime juice, soy sauce, sriracha, brown sugar, and sliced mushroom into broth. Bring to a simmer over medium heat and cook for 3 minutes. Add shrimp and cook 2-3 minutes longer, until shrimp is pink and fully cooked.
- Ladle soup over noodles and serve with cilantro and extra lime.