Roughly chop 2-3 tomatoes, 2 small onion, 2 garlic, some curry leaves and coriander leaves and transfer contents to a cooker.
Add 1.5 cup water and keep it for 2 whistles.
Open cooker when heat is released and let it cool down.
Transfer to a blender when completely cooled, and grind to a smooth texture.
Pour the grinded mix onto the same cooker or any other pan. If its very thick add some water. Do not add more of water as it may become very diluted and runny. It should be on the thicker side. Add salt and 1-2 tsp of rasam powder. (Rasam powder is nothing but cumin and pepper powder. If you do not have rasam powder, simply add cumin powder and pepper powder.) Bring it to boil. Optionally you can add butter too.
Enjoy hot tomato soup with some snacks.