Put the sweet potatoes in a medium-size saucepan, and cover with the water. Bring the water to a boil over medium-high heat, and cook until the potatoes are tender, about 12 to 15 minutes. Transfer the potatoes to a blender, adding just enough water to help it blend smoothly. Blend the mixture until there are no lumps, and the soup is creamy and smooth, and then pour the soup into a clean saucepan.
Pour the oil into a small skillet and place over medium-high heat. Add the ginger, and cook for 1 minute, and then add the fenugreek seeds, cumin seeds, fennel seeds, salt, pepper, and fenugreek leaves, if using and cook the spices for about 10 seconds. Add the mixture to the sweet potato soup, and mix until the spices are evenly distributed. Transfer the soup to warm soup bowls, and serve.