In a large pot, bring the oil to temperature over medium heat. Add the ginger, lemongrass, and curry paste and sauté for a couple of minutes.
Once the mixture is good and fragrant, pour in the chicken broth and whisk well to fully incorporate the curry paste. Then add the fish sauce and bring to a boil. Reduce the heat and simmer for 15 minutes.
Stir in the coconut milk and cooked chicken and allow the soup to simmer for another five minutes.
Stir in the lime juice, and then taste. You may or may not feel like it needs salt. Fish sauce is pretty salty all by itself. Add salt if needed and serve with chopped cilantro.
For a heartier meal, ladle the soup over a ½ cup of rice (I use brown, but you could go the traditional route as well and use white).