Season beef with salt and pepper.Dot with 2 tablespoon. butter.Melt remaining 2 tablespoon. butter in small roasting pan.Add carrot,leek and celery.Saute over low heat 8 minutes.Place beef in pan.Bake in preheated 500 degree oven 25 minutes.Cool in pan juices 1 hour. In medium skillet,heat oil and garlic.Cook 1 minute.Add bacon. Saute until barely crisp.Drain on paper towels.Drain all but 3 tablespoon. drippings.Add mushrooms.Cook over moderate heat 3 to 5 minutes.Remove,drain and set aside. Place cooled meat on cutting board,pouring pan juices into bowl. Add remaining ingredients to pan juices,blending well.Add bacon and mushrooms.Taste for seasoning.Cut wedge 3 cm wide and 3 cm deep along top length of beef.Remove long triangular wedge.Spoon stuffing evenly into cavity,filling with about 3 tablespoon. of stuffing.Replace wedge.Chill.To serve,cut beef into 2 cm slices.Accompany with remaining stuffing.Serves 6 to 8.