Defrost the fish and remove the skin and bones. Cut the fish into small pieces and drain as well as possible. Pulse in the food processor with parsley and salt.
Finely chop the onion, sauté in a frying pan with a little olive oil, and add the crumbled stock cube. Once the onion has softened, remove from the heat.
In a bowl, mix together the fish, the onion and one egg, and shape into two large (or several smaller) patties.
Place the flour, the egg and the breadcrumbs in three separate bowls. Dredge each patty first in the flour, then in the egg, and finally in the breadcrumbs, covering the whole surface. Repeat the process (double breading)
Fry in a little olive oil over a medium heat for 8-10 minutes on each side.
For the yoghurt sauce, whizz all the ingredients together in a food processor.
Wash the potatoes, cut into wedges and stir together with the spices and olive oil. Place in an ovenproof dish lined with baking parchment and bake for 30 minutes at 220 degrees Celsius.