Place the chopped squash pieces with 1/2 cup of water and turmeric in a microwave safe bowl and cook in the microwave for about 8-10 minutes, until the pieces are fork tender. You can also boil the pieces on stove top.
In heat 2 tsp oil in a pan, add urad dal, mustard and cumin seeds, hing and curry leaves. When the seeds splutter, add squash and saute for 2-3 minutes.
Add tamarind paste and 1/2 cup of water. Cover and cook on medium low flame for 5-6 minutes.
Add sambar powder and salt, cook for another 2-3 minutes. Garnish with coriander leaves and serve with rice.