Start by poaching the fish in boiling water for about 10 minutes until cooked through.
Drain the fish and let it cool before flaking it into small pieces.
In a large bowl, combine the flaked fish, mashed potatoes, chopped onion, garlic, and parsley.
Beat the egg and add it to the mixture. Season with salt and pepper.
Mix everything until well combined, ensuring there are no large chunks.
Shape the mixture into small patties, about the size of your palm.
Coat each patty in breadcrumbs, pressing lightly to adhere.
Heat oil in a frying pan over medium heat. Once hot, add the fish cakes.
Fry for about 4-5 minutes on each side until golden brown and crispy.
Remove from the pan and drain on paper towels before serving.