Rinse 1 cup masoor dal (the orange lentils), or split yellow moong dal or toor dal and place ithe lentils in a crockpot whose insides you have lightly oiled.
Add 3 cups water, put the crockpot on high and cook for two hours.
Stir, and check to see if more water is needed. Keep checking for water and stirring every 30 minutes until the lentils are cooked smooth.
When it is almost time to eat, in a large nonstick frying pan put the oil and mustard seeds. Heat over high heat until the mustard seeds have popped for about 10 seconds.
Quickly stir in the hing and curry leaves.
Add the tomatoes (optional) and serrano chilies. Stir-fry for about 4 minutes.
Add the spinach (this is optional). Stir-fry until the spinach is very tender, adding a little water if needed.
Stir in the turmeric, ground cumin and ground coriander.
Combine the spices and lentils either in the frying pan or the crockpot and mix until well blended. Check for water.
Add salt to taste and stir in the lemon juice. If desired, garnish with a little cilantro.
Variation: Instead of the spinach you can add 1 minced carrot that you have cooked in a bowl of water in the microwave for about 10 minutes, along with 1 large diced potato. When adding the cooked carrot and potato pieces to the fryiing pan, add 1/2 cup frozen green beans and 1/4 cup frozen peas. If you want to use fresh green beans, cook them in the microwave like you did the carrots and potato.