1 large yellow onion (250g)
2 tablespoons (30 ml.) olive oil
1 1/2 teaspoons sea salt, plus more to taste
2 large sweet potatos (1 1/2 pounds; 700 g)
1 large leek, white and light green parts (5 ounces; 140 g)
1 bunch spinach (8 ounces; 225 g)
1 large bunch green chard (12 ounces; 350 g)
3 tablespoons (30 g) chopped fresh ginger, plus more to taste
2 cups (500 ml) good-tasting vegetable broth
2-4 teaspoons fresh lemon juice
freshly ground black pepper
Chop the onion and cook it slowly in the olive oil with a sprinkle of salt, stirring now and then, over low heat until it is soft and golden, about half an hour.
Meanwhile, peel and dice the sweet potato and put it in a large soup pot with 4 cups (1 liter) water and a teaspoon of sea salt. Thoroughly wash the leek, spinach, and chard, chop them coarsely, and add them to the pot, along with the chopped ginger.
Bring the water to a boil, then lower the heat and simmer the soup, covered, for 20 minutes, or until the vegetables are completely tender. Add the caramelized onions when they are ready. When the vegetables are soft, add the vegetable broth (you can add less if you like a thicker soup) and decide whether you want your soup chunky, like this, or smooth. If the latter, puree the soup in a blender, in batches, or with an immersion blender until it is smooth.
Stir in 2 teaspoons of the lemon juice and a few grinds of black pepper. Taste, and correct the seasoning with additional salt or lemon juice.