Wash and pick over the lentils. Place in a saucepan and cover with water. Bring to a boil. Cook, covered, about 20 minutes. Add water if needed so the lentils stay covered.
Meanwhile, brown the onion in a large casserole. Stir in the garlic and coriander. Add the spinach and sauté 5 to 6 minutes, stirring frequently. Add the potatoes, lentils, and enough lentil cooking liquid to cover (for more broth, add more tap water). Season with salt and pepper to taste.
Bring to a boil, lower the heat, and cook at a simmer until potatoes are soft, about 20 minutes. For a thicker, more stew-like consistency, you can simmer up to one hour.
Immediately before serving, stir in the lemon juice. Taste and, if needed, add more salt and ground black pepper.