Pressure cook dal with enough water and 1/4 tsp turmeric powder till its soft and cooked properly. Mash the lentil and keep it aside.
Add 2 tsp oil in a wok and when its warm, add carrots and tomatoes and saute for few minutes.
Add salt to taste, cooked lentil and mix it well. If you feel its too thick, you can add some more water and adjust the consistency.
Add 1tsp oil in a small frying pan and when its warm, add mustard seeds. When mustard seeds splutter, add red chillies, a pinch of asafoetida, curry leaves and saute for few seconds. Add this in the Sambar and serve it hot with some rice or Idli or Dosa.