Add the mustard seeds. When they sputter, add the garlic.
Saute garlic over medium-low heat until it starts to turn lightly golden. Stay with it and don't let it burn.
Add the red pepper flakes, turmeric, and tomatoes and saute for a couple of minutes until the tomatoes start to break down.
Add the chopped spinach and saute until it wilts down, about two minutes.
Add the peanuts and the lentils and mix well.
Add three cups of water or enough water to drown the lentils. Add salt to taste and mix well.
If using a pressure cooker, cook for two whistles, or for six minutes after pressure builds up.
If cooking the spinach dal without a pressure cooker, bring the dal to a boil and then cover with a tight-fitting lid.
Lower the heat to a simmer and cook the dal for 30 minutes or until the dal is really soft. When cooking in a regular pot, you might need to check a couple of times to see if you need to add more water. Add hot water if required, instead of room temperature water, so you don't slow down the cooking process.
When the dal is cooked, I use a whisk to beat the dal five or six times. This makes the consistency of the dal smoother. In India, cooks use a wooden churn, called a mathani or a ravi, but I find a whisk serves the purpose just as well and is more easily found in most kitchens.