Place tenderloin on a rack in a shallow roasting pan, tucking small end of meat underneath. Bake, uncovered, at 425 degrees for 25 to 30 minutes. Remove from oven and let stand 30 minutes. Combine liverwurst spread, mushrooms and bourbon. Set aside.
Roll pastry to a 20 x 14 inch rectangle on a lightly floured surface. Spread 1/3 of liverwurst mixture over top of tenderloin. Place tenderloin lengthwise in middle of pastry, top side down. Spread remaining liverwurst mixture over sides of tenderloin. Bring sides of pastry up and overlap slightly to form a seam, trimming off excess pastry. Reserve all pastry trimmings. Trim ends of pastry to make even; fold over ends of pastry to seal. Invert roast.
Combine egg yolk and milk; brush evenly over pastry. Roll out pastry trimmings; cut into decorative shapes and arrange on top of pastry, as desired. Brush shapes with remaining yolk mixture. Bake, uncovered, in a lightly greased 13 x 9 x 2 inch pan at 425 degrees for 30 minutes. Let stand 10 minutes before slicing. Garnish with parsley. Yield: 12 to 15 servings.
