Rinse the fish fillets with cold water, pat dry, season with salt and pepper.
Peel, grate and salt the potatoes.
Cut the fish fillets into strips two fingers wide, turn in the flour, brush with mustard on both sides, cover with the grated potatoes and press down well.
Heat the butter in a non-stick pan and fry the fish fingers for about five minutes on each side until golden brown. The cooking time depends on the thickness of the fish fillet and the fish used.