Spread the lentils on a platter and pick them clean of any impurities. Rinse & drain.
Put the lentils and water in a large saucepan and place over a high heat. Bring to the boil, reduce the heat to medium, cover the pan, and boil gently for 30 - 45 minutes or until the lentils are tender.
While the lentils are cooking, wash the swiss chard, trim the dirty bottom ends of the stalks, if any, and chop into thin strips about 1/2-inch wide. Add to the lentils & simmer another 5-10 minutes until the chard has softened.
Peel the garlic cloves, chop them coarsely, and put them in a mortar. Add a generous pinch of salt to absorb the juices released from the crushed garlic and pound with a pestle until you have a smooth paste. You could squeeze the garlic with a garlic crusher but the texture and taste of pounded garlic is nicer.
Slowly incorporate the lemon juice into the garlic paste, then the olive oil.
When the lentils and chard are done, stir the garlic mixture into the soup. Season with salt to taste and simmer uncovered for 5 minutes.
Taste, adjust seasoning if necessary and serve at room temperature.