Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Heat oil in a large ovenproof pan. Dab fillets dry and fry them all around for 8-10 minutes. Season with salt and pepper.
In the meantime, mix butter with the whole mustard. Remove the fillets from the pan. Briefly fry the bread in hot frying fat on each side. Remove from the stove. Place the fillets on the slices of bread in the pan and spread with the mustard butter.
Bake in a hot oven for 12-15 minutes.
Wash the chives, dab dry and cut into fine rolls. Cut the fillets into larger pieces and arrange on the slices of bread. Sprinkle with chives and serve. Serve with a green salad.