Peel and chop onions. Wash leek well and chop. Mince garlic. Peel and mince ginger. Scrub and chop the sweet potatoes. Set aside.
Melt the fat in a large stockpot over medium-high heat. Add the onions and leek, 1/2 tsp. of salt, stir to coat, and let cook, stirring occasionally until softened and translucent, about 7-10 minutes. Add the garlic and ginger, stir well to coat, and cook for 2 minutes. Lastly, add the coriander, cumin, paprika, turmeric, and cayenne (if using). Stir to coat, and cook for another 2 minutes.
Add the sweet potatoes to the pot, stirring to coat. Add the vegetable broth and tamari, bring to a boil, reduce to simmer and cook, covered for 15 minutes (it’s okay if the vegetables are not fully submerged).
When the sweet potatoes are fork-tender, add the coconut milk and bring to a simmer, then turn of the heat and let cool for a few minutes. Transfer to a blender, add the lime juice and blend on high until completely smooth. Taste and adjust seasoning to suit your taste, adding the remaining 1½ tsp salt as needed. Add more water too thin, if necessary.
To serve, heat soup back (if necessary), garnish with cilantro and a sprinkle of cayenne or other chilies, if desired. Say thank you and enjoy. Store leftovers in the fridge for 5 days, or freeze for 4 months.