There are a lot of variations on the onion 'mix' depending on where you go. The onion raita is by definition onion in yoghurt with various spices. The onion in tomato ketchup is the bottom end ( in my opinion) of onion in a spiced tomato sauce. Usually it is just called onion salad, and is just that, onion a bit of salt, coriander leaf, chopped tomato and lemon juice. I have never seen it as a 'dish' on its own as such, it has always an accompaniment to poppadoms. Like Pete said, it is often given free in some form with takeaways or deliveries.
It would be interesting to see what other variations people have seen on their travels.