Lentils with Spinach and Lemon

1/4 kg. lentils, well washed
1/4 cup olive oil
1 large onion, sliced
3-4 cloves garlic, chopped finely
2 Tbs. fresh coriander, chopped
300gr. spinach leaves, well picked over and chopped
2 medium potatoes, peeled and sliced
1/4 cup lemon juice (or more, to taste)

Place the lentils in a saucepan and cover with water. Bring to the boil, cover and let cook for about 20 minutes. Heat the oil in a large casserole and brown the onion. Add the garlic and coriander, stir well and then add the spinach. Saute for 5-6 minutes, stirring frequently. Add the potatoes, lentils and enough of the lentil cooking liquor to cover. Season to taste with salt and pepper, bring to the boil, lower the flame and simmer until the mixture is thick and soupy (about 1 hour). Stir in the lemon juice. Serve hot or lukewarm.