Cook the Fish
Steam or poach the fish fillets until just cooked through (about 8–10 minutes).
Allow the fish to cool slightly, then flake it into small pieces using a fork, removing any bones.
Mix the Ingredients
In a large mixing bowl, combine the flaked fish, mashed potatoes, breadcrumbs, egg, parsley, green onion, garlic, lemon zest, Dijon mustard, salt, and pepper.
Mix gently until the ingredients are evenly combined.
Shape the Fish Cakes
Divide the mixture into 8 portions and shape each portion into a patty about 1/2 inch thick. If the mixture is sticky, lightly coat your hands with breadcrumbs.
Cook the Fish Cakes
Heat the olive oil in a large skillet over medium heat.
Place the fish cakes in the skillet and cook for 3–4 minutes per side until golden brown and crispy.
Serve and Enjoy
Transfer the fish cakes to a serving plate and serve warm with your favorite dipping sauce or a side of fresh greens.