Step 1Wash the greens thoroughly, then cut the chard and kale off their stems and slice the leaves. Combine the chard, kale, green onions and cilantro in a soup pot with the water and 1 teaspoon of salt. Peel the potato, or just scrub it well if you prefer, cut it into big pieces and add it to the pot. Bring the water to a boil, reduce the heat and let it simmer for about half an hour.
Step 2Meanwhile, heat 1 1/2 teaspoons of oil in a nonstick skillet over medium heat. Add the chopped onions and a sprinkle of salt and cook them over low heat until they are golden brown and soft. This will take up to 45 minutes; don't hurry, you only need to give them a stir once in a while, and it's the slow cooking that develops the sweetness. If you like, you can deglaze the pan at the end with 2 tablespoons of Marsala or Sherry. Increase the heat to medium, remove the skillet from the stove, and add the Marsala. Return it to the stove and cook the onions, stirring, 1 minute. Add the onions to the soup.
Step 3Put another teaspoon of oil in the skillet and cook the garlic over low heat, stirring, until it sizzles and smells great, 1 to 2 minutes. Add the garlic to the soup pot and simmer everything for a few minutes more.
Step 4At this point there won't be much liquid in the soup, so add enough broth-about 3 cups-to make the soup a soup. Coarsely puree the soup but don't over-process it. Anything with potatoes in it can get slimy if you work it too much.
Step 5Return the soup to the pot, bring it back to a simmer and taste. Add salt as needed, grind in a little black pepper, add the cayenne and the lemon juice. Stir well and taste again. Now you're on your own-correct the seasoning and serve big steaming bowls of green soup.