Heat the oil in a large saucepan over medium heat. Cook and stir the ginger, lemongrass and curry paste in the oil for 1 minute. Slowly pour the chicken stock over the mixture, stirring continually. Stir in the fish sauce and brown sugar then simmer for 15 minutes.
Stir in the coconut milk and mushrooms then cook and stir until the mushrooms are soft, about 5 minutes. Add the prawns then cook for 5 minutes or until pink and opaque.
Remove from heat and stir in the lime juice and salt then garnish with fresh coriander.