If using raw fish: poach or steam 8–10 oz fish in simmering salted water for 5–8 minutes until opaque; drain and cool. If using leftover cooked fish, flake into small pieces, removing bones and skin. Mash hot cooked potato until smooth; allow to cool slightly.

In a large bowl, combine 1.5 cups flaked cooked fish and 1 cup mashed potato. Break up the fish so there are small flakes distributed through the potato.

Add 2 tbsp finely chopped green onion, 2 tbsp chopped parsley, 1 tsp Dijon mustard, 1 tsp lemon juice, 1 tsp lemon zest, 1 tsp salt, 1/2 tsp black pepper, and 1 lightly beaten egg. Mix gently but thoroughly until just combined. Avoid overworking to keep texture delicate.

Taste a small pinch of the mixture (optional) and adjust seasoning with additional salt, pepper, or lemon juice if needed.

Divide the mixture into 6 equal portions (about 3–3.5 oz / 85–100 g each). Shape each portion into a compact patty about 2.5–3 inches (6–7.5 cm) wide and 3/4 inch (2 cm) thick. Place patties on a tray and chill in the refrigerator for 15–20 minutes to firm up; this helps them hold together during cooking.

Coating

Set up a dredging station with three shallow bowls: bowl 1 with 1/2 cup flour, bowl 2 with 1 beaten egg, bowl 3 with 1 cup breadcrumbs mixed with 1/4 tsp salt and 1/4 tsp pepper.

Lightly coat each chilled patty in flour, shaking off excess. Dip into the beaten egg, letting excess drip back into the bowl, then press into the breadcrumbs to coat evenly on both sides. Place coated patties on a clean plate and let rest 5 minutes to help the coating adhere.

To cook & finish

Heat a large nonstick or cast-iron skillet over medium heat. Add 3 tbsp neutral oil and heat until shimmering but not smoking (about 325–350°F / 160–175°C surface temperature).

Add patties to the pan without crowding (work in batches if necessary). Cook for 3–4 minutes on the first side until golden brown and crisp. Carefully flip and cook 2.5–3 minutes more until the second side is golden and the patties are cooked through (internal temp ~ 140–145°F / 60–63°C).

If using, add 1 tsp butter to the pan in the last 30 seconds and spoon the foaming butter over the cakes for extra flavor.

Transfer cooked fish cakes to a wire rack or paper towel-lined plate to drain briefly. Keep warm in a low oven (200°F / 95°C) while you cook remaining patties.

Serve fish cakes hot with lemon wedges and optional tartar or lemon mayo on the side.

Optional tartar or lemon mayo

While patties chill or cook, combine 1/4 cup mayonnaise, 1 tbsp minced cornichons or dill pickles, 1 tsp chopped capers, 1/2 tsp lemon juice, and 1 tsp minced fresh dill or parsley in a small bowl. Season with salt and pepper to taste. Refrigerate until ready to serve.