- In a pot of salted boiling water, cook the potatoes for about 15 minutes or until they are tender. Drain. Return to the pot. Using a potato masher, purée the potatoes. Season with salt and pepper. You should have about 1 ½ cups (375 ml) of purée. Set aside.
- In a non-stick skillet over medium heat, cook the fish in the butter for 2 to 3 minutes for each side, depending on the thickness of the filet. Season with salt and pepper. Transfer the cooked fish to a plate and let cool. Using a fork, break up the fish into small flakes. Add to the potato purée.
- Add the egg and mustard to the potato-fish mixture and mix well.
- Using a ¼ cup (60 ml) ice cream scoop, shape the mixture into balls. You should get about 8 balls. Flatten with your hands to form into cakes.
- Place the breadcrumbs in a bowl. Using your hands, dredge the cakes in the breadcrumbs, lightly pressing them into the crumbs so they are fully covered.
- In a non-stick skillet, heat the oil over medium-high heat. Cook half the fish cakes in the oil for 2 to 3 minutes on each site or until they are nicely golden on both sides. Watch out for splattering. Drain on paper towels. Repeat with the remaining fish cakes, adding more oil as needed.