Coat each of the beef fillets with a little mustard. Crush the peppercorns in a mortar and sprinkle over the beef fillets. Clean the young onions, remove the coarse green, wash and cut into strips or wedges. Melt half of the butter in a saucepan and slowly sauté the young onions in it until soft. Season with salt and pepper. Meanwhile, in a frying pan, heat the oil with the remaining butter and fry the beef fillets in it, first until hot and then slowly until pink. While doing so, keep basting with the gravy. Arrange the finished beef fillets on preheated plates. Cover with the stewed onions and drizzle the remaining gravy over them.