Heat oil in a heavy pan over medium heat. Sauté onion for 10 minutes or so, until it begins to turn golden. Add minced garlic and sauté for another minute or so.
Add dhal and water to the saucepan. Bring mixture to the boil. Cover, lower heat and simmer about 35 minutes, until lentils are soft (this may take less time, depending on your water and the lentils).
Meanwhile cook the spinach in microwave till it totally wilts. Add spinach, salt and cumin to the saucepan. Cover and simmer until heated through, about ten minutes. Grind in plenty of pepper to taste.