Heat olive oil in a medium saucepan. Add the onions, ginger and garlic. Cook over medium heat until softened, about 3-4 minutes.
Stir in Thai red curry paste, and cook several minutes or until fragrant.
Add broth, lentils, sweet potato, lemongrass, kaffir lime leaves and turmeric. Bring to a boil, then reduce heat to medium-low, cover and cook about 20-25 minutes or until lentils and sweet potatoes are soft.
Remove lemongrass and kaffir lime leaves. Add salt and stir in coconut milk. Cook five more minutes. Puree if desired.
Garnish with kaffir lime leaf slivers or fresh cilantro, and a drizzle of coconut milk.
I use homemade vegetable stock such as Magic Mineral Broth for this soup because it has extra nutrients in it.