Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Dab fillets dry, season with salt and pepper. Heat oil in an ovenproof pan. Fry the fillets thoroughly all around.
Remove and set aside.
Briefly toast slices of bread in hot frying fat from both sides. Wash the chives, shake dry and cut into fine rolls. Mix butter, whole mustard and 2 tbsp. chives.
Cut the fillets into 12 medallions and spread them on the bread slices. Spread with the mustard mixture and gratinate in a hot oven for about 10 minutes.