Place tenderloin, fat-side up, on rack in shallow roasting pan; roast in preheated 450 F. oven 25 minutes, or until meat thermometer registers 130 F. Remove meat from oven; cool.

Meanwhile, saut‚ mushrooms and onion in butter and olilve oil; blend in pat‚ and parsley, blending well. Roll pastry on lightly floured board into two 12x8-inch rectangles; spread half of pate mixture evenly on each rectangle, leaving 1-inch margin on all sides. Center tenderloin, fat-side up, on first rectangle so pate side is inside; cover with remaining rectangle with pate side next to meat. Moisten edges of pastry with water; overlap and press to securely seal edges. (Roll out remaining dough and cut decorative shapes for top.) Add 1 tablespoon water to egg white for egg wash; beat lightly and spread evenly over entire top of pastry. Bake in preheated 400 F. oven 40 minutes, or until pastry is golden. Slice and serve with B‚arnaise Sauce.

Sauce: Combine shallots, parsley, tarragon, chervil and vinegar in water; bring to a boil and simmer 3 minutes. Drain. Add liquid, a few drops at a time, to egg yolks in top of double boiler, stirring constantly with wire whisk. Cook over hot water, stirring constantly, until mixture thickens. Add butter, 1 tablespoon at a time, blending well after each addition. Season to taste with salt and cayenne. Serve with Beef Wellington.