a lemon-sized piece of tamarind pulp, soaked in 2 cups hot water, strained, squeezed to remove juice.

For tempering:
3 tbsp sesame oil
2 red chilies, halved
1 tsp brown mustard seeds
1/2 tsp asafoetida powder
1/2 tsp fenugreek seeds
1 tsp toor dal/red gram dal/pigeon peas
1 tsp chana dal/yellow split peas
1 tsp urad dal/black gram dal
a few curry leaves

150g/5oz shallots, peeled and chopped, or green onions
3 tsp sambar powder
2 tbsp powdered jaggery
2 tbsp besan (chickpea flour)
1/2 cup water

Heat oil. Add chilies, mustard seeds, asafoetida, fenugreek. When mustard seeds splutter, add the dals and curry leaves. Saute until golden.
Add shallots, saute 2 minutes. Add sambar powder, saute 1 minute. Add tamarind juice, salt, and jaggery, saute 10 minutes. Make a batter with besan and water, add to sambar, boil 2 minutes.

Source: Dakshin, Vegetarian Cuisine from South India by Chandra Padmanabhan