Splutter mustard seeds in little oil, saute garlic, jeera, chilliesand turmeric powder.
Add the onions, saute until golden brown, then the tomatoes if adding.
Boil moong dal separately and add to this mixture or you can soak the dal for an hour and add it to the pan with water, let it cook.
Now add the spinach and salt, close with a lid and let it cook. You may let it become thick if using as a side dish or leave the dal a bit liquidy if eating with rice.