Thoroughly wash and coarsely chop the leeks, using only the white and light green part, and chop the onion. Heat the olive oil in a non-stick pan and start sautéing the onions, with a sprinkle of salt. When they are translucent and soft, add the leeks and keep cooking, stirring often, until all the vegetables are golden, about 20 minutes.
Meanwhile, peel the sweet potatoes, scrub the small Yukon gold or white potato, and cut them all in 1/2 inch dice. Trim the thick stems from the kale, and cut the greens into one-inch strips, or chop them very coarsely. Combine the sweet potatoes and kale in a soup pot with 5 cups (1 1/4 liter) cold water and a teaspoon of salt, bring to a boil, then lower the heat and simmer for about fifteen minutes.
Add the sautéed leeks and onions to the pot, along with the sliced green onions, cilantro, and a lot of fresh ground black pepper. Add as much of the vegetable broth as you need to give the soup a nice consistency - this is a hearty soup, but not a stew, and it should pour easily from a ladle. Simmer the soup gently, covered, for about ten more minutes.
Lightly toast the cumin seed in a dry pan, just until it is fragrant, and grind it in a mortar or spice grinder. Stir the cumin seed and a spoonful of lemon juice into the soup, and taste. Add more salt, pepper or lemon juice as needed, and finish with a pinch of cayenne or any red pepper.
Ladle the soup into warm bowls, and garnish each bowl with a swirl of fruity olive oil. If you like cheese, a heaping spoonful of tangy crumbled feta cheese dropped on top of each serving is fantastic.
Yield: 6 generous servings