Heat 2 tbsp of oil in a separate pan. Add mustard seeds, cumin seeds, and fenugreek seeds.
Once the seeds start to pop, add curry leaves and asafetida.
Add finely chopped onions along with ½ tsp of salt. Saute on medium heat till they are translucent.
Add chopped tomatoes and saute till they become mushy and start to loose water.
Add ¼ cup of sambar spice mix, thai green chili & ginger paste, red chili powder, turmeric powder, cumin-coriander powder, and 3 tsp of salt. Mix well and cook it until the oil starts to separate, do not burn it.
Add this onion/tomato spice mix to blended dal we prepared earlier. Mix very well and bring it to boil.
While the sambar is simmering, add finely chopped cucumber, cilantro and lemon juice. Mix well.
Serve it with idli/vada/dosa or just plain rice!