Cook and flake 400 g white fish; remove bones and skin.
Mash 250 g cooked potato or place 1 cup breadcrumbs in a bowl.
Finely chop 1 small onion and sauté in 1 tbsp oil until soft; cool slightly.
In a large bowl combine flaked fish, mashed potato/breadcrumbs and sautéed onion.
Add 1 beaten egg, 1 tsp lemon zest, 2 tbsp chopped parsley, 1 tsp mustard, salt and pepper.
Mix gently until ingredients bind; add 2 tbsp mayo or yogurt if mixture seems dry.
Shape mixture into 8 evenly sized patties, pressing firmly to hold.
Heat 3–4 tbsp oil in a nonstick skillet over medium heat.
Fry fish cakes 3–4 minutes per side until golden brown and heated through.
Transfer to paper towel to drain; rest 1–2 minutes before serving.