Separate the lettuce leaves, wash very well and chop in small to medium sized pieces. Keep the chopped lettuce aside.
Wash and chop the tomatoes too, keep this aside with chopped lettuce leaves.
Transfer the chopped lettuce and tomatoes to a pressure cooker.
Wash the dal and add it also to the pressure cooker.
Add salt, turmeric powder, red chili powder, coriander powder, fenugreek powder (you can use ready made sambar powder instead of adding spices separately or use a mix of both if you like) and also water to the pressure cooker, mix everything well using a large spoon.
Close the pressure cooker with lid and safety valve, cook for about 12 to 15 minutes in medium heat or until 3 whistles come. Wait for the pressure cooker to cool down.
Once the pressure cooker is cooled enough, which will take few minutes, open the cooker and see if the lettuce and dal is cooked and soft enough. Just mix all contents very well and if you wish to thicken it more, you can heat the contents for another 2 to 3 minutes. Now the sambar is ready, let us prepare the seasoning.
To prepare the seasoning for sambar, heat the oil in a small pan and add the mustard seeds.
When the mustard seeds splutter, add the broken dry red chilies and curry leaves, fry for a few seconds.
Pour this prepared seasoning over the sambar, gently mix in.