Wash the greens thoroughly, trim off their stems, and slice the leaves. Combine the chard/spinach, kale, green onions, and cilantro in a large soup pot with 3 cups of water and a teaspoon of salt. Peel the potato, or just scrub it well if you prefer, cut into small pieces, and add it to the pot. Bring the water to a boil, turn down the flame to low, cover the pot, and let the soup simmer for 30 minutes.
Meanwhile, chop the onion, heat a tablespoon of olive oil in a skillet, and cook the onion with a small sprinkle of salt over a medium flame until it is golden brown and soft. This is take up to 30 minutes. Don't hurry; give it a stir once in a while and let the slow cooking develop the onion's sweetness. If you like, you can deglaze the pan at the end with a bit of Marsala or sherry.
Add the caramelized onion to the soup. Put the remaining 1/2 tablespoon oil in the pan and stir the chopped garlic in it for just a couple of minutes, until it sizzles and smells great. Add the garlic to the pot and simmer the soup for 10 more minutes.
Add enough broth to make the soup a soup - it should pour easily from a ladle - and puree in a blender, in batches, or use an immersion blender. Don't overprocess; potatoes can turn gummy if you work them too much.
Return the soup to the pot, bring it back to a simmer, and taste. Add a pinch more salt if needed, grind in some black pepper, and add a pinch of cayenne and a tablespoon of lemon juice. Stir well and taste again.
Garnish is a drizzle of olive oil.