In a large pot over high heat, saute the onion and oil until fragrant.
Add the garlic and curry paste and saute for 1 minute until the oil from the curry paste has been released.
Add the carrots and potatoes and saute for 5 minutes or until slightly browned.
Add the lentils, vegetable broth and half the coriander. Boil for a few minutes then reduce the heat to low. Cover the pot with a lid and simmer for 20 minutes. The soup is ready when the potatoes and carrots are tender (insert a knife into a piece to test its texture). If you'd like a thicker soup, cook it for longer. If you'd like a runnier soup, add more vegetable broth.
If desired, blend half of the soup with a stick blender or stand blender. The soup will be very hot so be careful! Season with salt and pepper, to taste.
Divide the soup into serving bowls. Top with additional coriander and season with red pepper flakes and lime juice if desired. Serve with or without crusty bread.
