Wash moong dal and tomatoes. Halve the tomatoes. Place both dal and tomatoes in a pressure cooker and add 1-1/4 cups water and pressure cook for 2 whistles. Wait for the pressure to release.
After you open the cooker lid, you can find that the skin of the tomatoes have come loose. Just peel them away and discard. Mash the dal along with tomatoes with a masher or pappu guthi and keep aside.
Chop the onions finely. Cut the green chilies into 3 or 4 pieces.
Heat a pan with oil. Add mustard seeds and let it splutter. Add onions, green chilies, curry leaves and sauté till the onions turn translucent.
Add the mashed tomato and dal mixture to the pan and further add 3/4 to 1 cup water to the consistency you want the gotsu to be.
Add salt, turmeric powder, sambar powder, mix well and let it come to a boil.
Simmer the flame, add coriander leaves and let it cook for 4 to 5 mins.
Onion tomato gotsu is ready.
Serve piping hot poured generously over spongy idlis or crispy dosas.