Heat a large, heavy-bottomed soup pot over medium heat. Add coconut oil and let it melt. Sauté diced onion until translucent and soft, about 5 minutes. Add dried chili flakes, ground coriander, ground cumin, and ground turmeric, sauté for about a minute. Add minced ginger and garlic, cooking for another minute, then season with salt and black pepper.
Add peeled and diced sweet potatoes and dry brown lentils to the pot. Stir to coat in spices. Season with salt and pepper. Pour in vegetable stock, scraping up any browned bits from the bottom. Bring to a boil.
Reduce heat to let it simmer gently for about 30 minutes or until sweet potatoes are tender and lentils are cooked through. The liquid should reduce by almost a third.
Stir in coconut milk and chopped kale. Simmer for an additional 3-4 minutes until kale wilts. Adjust seasoning if needed before serving.
Serve hot, garnished with chopped cilantro, extra chili flakes, lime wedges, or nigella seeds.