In a large pot or Dutch oven, heat 1 tablespoon of coconut oil over medium heat. Add the diced yellow onion and sauté for about 5 minutes until soft and translucent. Stir in the dried chili flakes, ground coriander, ground cumin, and ground turmeric, cooking for an additional 1-2 minutes until fragrant.
Add the minced ginger and garlic to the pot. Cook for another 2-3 minutes, stirring frequently to avoid burning the garlic.
Add the diced sweet potatoes and dry brown lentils to the pot. Stir well to coat them with the aromatic mixture, allowing the flavors to meld for about 2 minutes.
Carefully add the vegetable stock and the can of full-fat coconut milk to the pot. Bring the mixture to a gentle simmer, stirring occasionally. Season with salt and black pepper to taste.
Cover the pot and let the stew simmer for about 25-30 minutes, or until the sweet potatoes and lentils are tender. Stir occasionally to prevent sticking.
Once the sweet potatoes and lentils are cooked, add the chopped kale to the pot. Stir well and let it cook for another 5 minutes until the kale is wilted and tender.
Ladle the stew into bowls and garnish with chopped cilantro, a sprinkle of extra chili flakes, lime wedges, and nigella seeds for an added crunch and flavor.