Heat a large, heavy-bottomed soup pot over medium heat. Add the coconut oil to the pot and let it melt. Add the onions to the pot and stir. Saute the onions, stirring occasionally, until translucent and quite soft, about 5 minutes. Add the chili flakes, coriander, cumin, and turmeric. Saute spices until very fragrant, about 1 minute. Add the ginger and garlic to the pot and cook for another minute. Add a pinch of salt and pepper.
Add the sweet potatoes to the pot and stir to coat in the spices. Add the lentils to the pot and stir once more. Season everything liberally with salt and pepper. Add the vegetable stock and stir, scraping up any browned bits on the bottom of the pot. Place the lid on top of the pot and bring to a boil.
Once the stew is boiling, lower the heat to a simmer and set the lid slightly askew on top of the pot so that there’s a couple inches available for steam to escape. Simmer until the sweet potatoes are almost falling apart and the lentils are tender, about 30 minutes. The liquid should also be reduced by almost a third.
Add the coconut milk and kale to the pot and stir. Place the lid on top and continue to simmer the stew until the kale is wilted and bright green, about 3-4 minutes. Season the sweet potato and coconut milk stew again with salt and pepper. Bring the stew back up to a strong simmer and check for seasoning. Adjust with more salt, pepper, chili etc. if necessary.
Serve the sweet potato and coconut milk stew hot with chopped cilantro, extra chili flakes, lime wedges, and Nigella seeds (if using).