Heat olive oil in a large saucepan over medium heat.
Add the onion and cook, stirring occasionally, until translucent and cooked through (roughly 5-7 minutes).
Add the garlic and ginger, and cook for one minute.
Add the curry paste and stir until onions/garlic/ginger are coated and you can smell the curry paste, 30 seconds-1 minute.
Add the sweet potato, carrots, stock, lentils, fish sauce, and salt. Cover, bring to a boil, reduce heat, and simmer for 20 minutes, until lentils and sweet potato are cooked through.
Using an immersion blender, puree the soup until smooth and creamy. Stir in the coconut milk and lime juice.