Discard the brine from the can, transfer the mackerel to a plate, debone the fish and transfer the flesh in a mixing bowl, add chopped onions, ginger garlic paste, red chilli powder, turmeric powder, black pepper powder, vinegar, and egg yolk.
Add the tomato ketchup. Mix everything well. Take some mixture, and try to form elongated croquettes, or any shape as you desire. If they hold their shape, continue with the crumb coating, if not, dip 1-2 slices of bread in water, squeeze the water out, and add this bread to the mixture, mix well. You should be able to form the croquettes now. Alternatively, you can also use boiled potatoes. My mixture held well, so I didn't need to add anything for binding. You should be able to get about 8 elongated croquettes with these measurements.
Beat the egg white in a bowl with little salt. Place breadcrumbs/semolina in another bowl, season with little salt. Now dip the croquette in egg white and then breadcrumbs. Coat all the croquettes in this manner.
Heat oil on medium heat in a skillet, fry the croquettes until they are golden brown from all sides, turning sides as required. Serve hot.