Season beef with salt and pepper.Dot with 2 tbsp. butter.Melt remaining 2 tbsp. 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 tbsp. 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 1" wide and 1" deep along top length of beef.Remove long triangular wedge.Spoon stuffing evenly into cavity,filling with about 3 tbsp. of stuffing.Replace wedge.Chill.To serve,cut beef into 3/4" slices.Accompany with remaining stuffing.Serves 6 to 8.