Cover the lentils with water, bring to a boil, cover with a lid and simmer until the lentils are tender, about 1 hour. I do this in a pressure cooker, but you can do this on the stove-top or in the microwave.
Heat the oil in a saucepan.
Add the cumin seeds and, when they sputter, add the sliced onions.
Saute the onions until translucent, then add the garlic. Stir-fry for another minute.
Add the spinach and saute for another 3-4 minutes. You want the spinach to have completely thawed before adding the remaining ingredients.
Add tomatoes, chilli powder, turmeric and mustard-seed powder. Stir together for about a minute.
Add the lentils-spinach mixture and stir in.
Add salt to taste. Bring the curry to a boil, then lower the heat and simmer for five minutes.
Serve hot with rice or chapatis.