Melt the butter in a large pot over medium heat. Add the onion, garlic and beet stems. Saute until onion is softened and fragrant.
Add the sweet potato, lentils and spices. Cook, stirring often for about 5 minutes. Add the broth and allow to simmer for 10 minutes, until sweet potatoes are tender.
Add the coconut milk and the beet leaves, and simmer for another 5 minutes, until leaves are bright green and coconut milk is incorporated. Enjoy hot with naan bread or a crusty french loaf.