After washing the fish pieces, cover the fish pieces with some salt and set aside.
On a pan, heat oil. Add onions and fry until the onions turn slightly golden. Now add the salt, coriander and red chilli powder. Stir until the oil starts to float on top.
Add the fish pieces and mustard paste, and cook for a while.
Add the green chillies,and yoghurt. Sprinkle the sugar and stir for a bit. While stirring make sure the fish pieces do not break. Cook until the water of the yoghurt dries up, and the dish turns gravy.
On a separate pan fry the one handful of thinly sliced onions until the onions turn dark golden brown. Sprinkle the fried onions ( beresta) on top of the Doi Ilish when it is being served.