This is a legacy, traditional recipe from Samaithu Par by Meenakshi Ammal. I have followed her recipe exactly, except one step. She adds lemon juice at the end, which I skipped.
This is a simple recipe where you don't need tamarind or even prepapred rasam powder, and can be ready in a few minutes. Fresh tomatoes are recommended, although canned whole peeled tomatoes can be used in a pinch.