Heat olive oil in a medium pot over medium-low heat.  Add the onions and a pinch of salt and stir frequently until the onions are soft and golden brown, 30 to 40 minutes. Meanwhile, place the sweet potatoes in a large pot with 3 ½ cups water, ½ teaspoon salt, and the sage.  Bring to a boil.  Lower the heat and simmer, covered, about 10 minutes. Wash the kale and chard, trim away the tough stems, and chop.  Add to sweet potatoes along with garlic and vegetable broth.  Cover and simmer gently for 20 minutes.  Add the onions to the soup and let cool slightly. Puree the mixture in a blender, in batches, and return the pureed soup to the pot.  Season to taste with salt, pepper, and lemon juice.  To serve, drizzle olive oil over individual bowls of soup.