Bring the lentils and water to a boil in a large soup pot, then reduce heat and simmer. When the lentils have cooked for about 40 minutes, saute the onion and garlic in the olive oil until the onion is soft, then add the mixture to the lentils, along with the salt and plenty of black pepper. Simmer for another 30 minutes or so. Just before serving, add about half the lemon juice, then taste to see whether you want to add the rest. Add more salt and/or pepper if you like.
This is a very simple and soothing soup.