I combined dal and veggies along with 5-6 cups water, ½ tsp turmeric, sambar powder, chili powder and salt in the slow cooker. Cooked on Low for about 6 hours (or on High for 4hrs).
So by the time we came home, dal & veggies were done, then I added tamarind extract and adjusted the seasonings. Cooked for another 30 minutes on High.
When there was about 10 minutes left for cooking, I made the tadka in 2tbsp oil, added it to slow cooker and let it cook.