Combine lentils, onion, green chilies, veggies, tomato puree, sambar powder, turmeric, salt and water in the slow cooker. Cook on Low for about 6 hours or on High for 4 hours.
By the end of the cooking time, dal & veggies should be cooked through. Stir in the tamarind extract. Taste and adjust the seasonings. Cook for another 30 minutes on High.
Heat oil in a small saucepan, add the mustard, cumin seeds, dry red chilies and cook till the seeds start to splutter. Add the curry leaves and cook for another minute. Pour the tempering in sambar and mix well. Serve hot with rice or idli.
Notes
You can use moong dal instead of red lentils to make sambar.
If you don't have canned tomato puree on hand, blend 2~3 fresh tomatoes and use.
You can add any combination of veggies. Some of my favorites are onion, bottle gourd or zucchini, potato, carrot and bell peppers. Choose veggies that cook easily.
Store leftover sambar in an airtight container for up to 3 days in the fridge. It might thicken as it sits, so add some water to loosen and adjust seasoning.
You can also freeze sambar for up to 2 months. Thaw in the fridge overnight and then reheat either in the microwave or on the stove.