In a pot, boil the lentils with 6 to 7 cups of water over medium heat for about 40 min.

Meanwhile, put the oil in a small pan over medium heat and fry the onion and garlic for 5 min or until they become golden in color.

When the lentils are cooked, add the onion and garlic mixture, potato cubes, spinach, salt and 4 cups of water and stir.

Cover the pot and leave on medium heat for 40 min or until the potato and spinach are well cooked. Add the lemon juice after removing from heat.

Serve hot.

Note: Depending on the type of lentils, you might need more or less water to cook. Small-sized lentils require more water.