Cook the fish fillets by steaming or poaching until flaky, about 10 minutes. Cool and flake into pieces.
Boil chopped potatoes in salted water until tender, about 15 minutes. Drain and mash until smooth.
Sauté the chopped onion in a skillet with oil over medium heat until translucent, about 5 minutes. Cool slightly.
In a bowl, combine flaked fish, mashed potatoes, sautéed onions, herbs, beaten egg, salt, and pepper. Mix gently.
Form the mixture into patties about half an inch thick and coat each with panko breadcrumbs.
Heat oil in a frying pan over medium heat and fry the patties for about 4 minutes per side until golden brown. Drain on paper towels.
