Rinse lentils and place in a large pot with the water. Bring the lentils to a boil, add the salt, and simmer until the lentils are almost tender.
Saute the onion and garlic in the oil until they are tender, and then add the potatoes, 2 T of the lemon juice, and the coriander.
Add the onion mixture to the lentils and water, add the spinach and simmer for 20 min or until the potatoes are tender.
Before serving, add the remaining 2 T of lemon juice.