We just blanch onions and peppers in water with haldi, salt and panch phoran. That gives the onions a yellow colour. We blanch them for 10 mins, and then for dishes like jal frezi and dopiaza we also add raw onion about 2 min before serving. That way you get the mild blanched veg plus a sweet, crispy and punchy onion crunch.
God I love curry - if there's a zillion ways of doing it, there's always a zillion-and-oneth!
Tim