Preheat oven to 350 F. Pierce sweet potatoes with a fork and bake for 50-60 minutes until soft.
Combine lentils, lemongrass, salt and 8 cups of water in a dutch oven and bring to a boil. Reduce heat to low, cover pot and cook for 15-20 minutes until lentils are very soft. Remove from heat. Remove and discard lemongrass.
Heat the oil in a pan over medium heat. Add onion, red pepper and carrots and saute about 15 minutes. Add ginger and garlic and continue to saute for 5 minutes. Add spices and saute an additional 3 minutes. Add to dutch oven with lentils.
When sweet potatoes are ready - peel and add flesh to dutch oven with lentils and vegetables.
Blend thoroughly with a hand-held immersion blender - or puree in batches in a blender.
Return to pot and add 1/2 Cup water and 1/4 Cup lemon juice.