This is a basic onion chutney that goes really well with idli, dosai, upma, rice, even rotis. The best version of this onion chutney is one I remember from one of those hole in the wall places in Pondicherry, from my childhood.
It was called "Mani Iyer Cafe" and it had those simple benches and chairs you see in simple roadside hotels or what they call "mess" in India. Mani Iyer cafe served the best, softest, steaming idlis with a spicy onion chutney that I used to absolutely love. The flavor was enhanced by the banana leaf it was served on. I've been trying to recreate that for a long time now and this comes as close to my memory as it can! I served this with a simple dosa called "neer dosa" and it was so good! (Recipe to follow soon)