Remove the stems from the Swiss chard and chop the leaves. Wash thoroughly. Sort through the lentils to remove any debris or stones, then rinse well. Chop the onions into medium-sized cubes. Squeeze the lemons for juice.

In a large pot, combine water, Swiss chard, and lentils. Bring to a boil, then add salt and reduce the heat. Simmer until the ingredients are cooked through. In a skillet, sauté the onions in oil until lightly browned, then add them to the pot. Add the lemon juice and let everything boil together for 5 minutes, then turn off the heat. The soup is now ready.

Serve warm or cold, as you prefer. Enjoy!