Heat oil in a heavy-bottomed soup pot over medium heat. Add onion and 1 tsp salt. Cook the onions until translucent. Add minced garlic, ginger, lemongrass, curry paste and coriander; sauté and stir for about 5 minutes. Stir in the chopped carrots and remaining salt. Sauté for several minutes, then add vegetable stock. Bring to a boil and simmer until the carrots are soft, about 15-20 minutes.
Next, puree the soup in a blender until smooth. You may need to puree the soup in several batches. Whisk in coconut milk and red pepper flakes.
Just before serving, add the lemon zest. Garnish each bowl with fresh cilantro.