Melt the butter in a large, heavy bottomed pot over medium-high heat. Place the chopped sweet potatoes, carrots, apple, and onion in the pot. Stir and cook until the onions are translucent, about 10 minutes.
2
Stir in the lentils, ginger, spices and broth. Bring to a boil over high heat, then reduce to medium-low, cover, and simmer until the lentils and vegetables are soft, about 30 minutes.
3
Working in batches, pour the soup into a blender, filling the pitcher no more than halfway full. Hold down the lid of the blender with a folded kitchen towel, and carefully pulse a few quick pulses to get the soup moving before leaving it on to puree. Puree in batches until smooth and pour into a clean pot. (If you have a stick blender, you can puree the soup right in the cooking pot.)
4
Return puree to cooking pot and bring back to a simmer over medium-high heat, about 10 minutes. Add water as needed to thin the soup to your preferred consistency.
5
Garnish with plain yogurt or cumbled feta cheese (not included in nutritional information.)
