The simplicity of a rasam is a decoy for its depth. At first sip, you may only discern the faint sweetness of a ripe tomato. Then comes the punch of tamarind. You reel momentarily from this affront, but you will soon be soothed by the nutty richness of mustard seeds fried in ghee — called the thalippu, or tempering that crowns this trellis of flavor.
For eons, South Indians of all stripes have claimed an intimate understanding of rasam, a broth (not unlike a stock) that teases complexity out of even the most minimal ingredients. At its simplest, this could mean a tomato or two, or a knob of dried tamarind and a scattering of spices — all allowed to commingle until their flavors merge into a cohesive whole.