Soak the toor dal (pigeon peas) for 4 hours before preparing the soup.
If you don't have 4 hours, boil the toor dal 4 minutes. Cover the pot and soak for 20 minutes.
Start the Instant Pot in sauté mode and heat oil in it. Add all the tempering ingredients.
When the mustard seeds start to sputter, add the chopped onions and sauté for 2 minutes until translucent.
Add the ginger and garlic pastes. Sauté again for a minute.
Add turmeric, sambar masala powder, green chilies (if using), black salt and sauté for another 2 minutes.
Add tomatoes and sauté for a few minutes until well combined.
Add all the vegetables, toor dal, water and salt to the Instant Pot.
Change the Instant Pot setting to manual or pressure cook mode for 7 minutes at high pressure with vent in sealing position.
When the Instant Pot beeps, let the pressure release naturally. Mix in the lemon juice, stirring to incorporate.
Stovetop Pressure Cooker Method
Follow the same steps as above in a stovetop pressure cooker. Cook on medium-high heat for 3 whistles, then turn off the flame and let the pressure release naturally. Stir in lemon juice in the sambar so it is mixed well.
Note: The Indian spices, curry leaves, asafoetida, black salt, and toor daal are available at Indian Grocery Stores
White gourd is also known as wax gourd, ash gourd, winter gourd, tallow gourd, as pumpkin, winter melon, and Chinese preserving melon. It is a vine grown for its very large fruit, eaten as a vegetable when mature. It has a very bland taste and can be substituted with chayote, zucchini, bottle gourd, or fuzzy melon.
For a traditional sambar soup, besides the tomatoes and onions, you can select any three vegetables that you like. Keeping your vegetable choices to three maintains the balance of the soup. Here are some options:
okra, vegetable drumstick, peas, French peas, golden pumpkin, yellow cucumber, eggplant, etc.
For a creamy base, remove half of the soup to a blender. Blend until smooth and return to the pot. Take caution when blending the soup as it is very hot.