1For marinade, in a heavy self-sealing plastic bag, combine wine, tarragon, and garlic. Place tenderloin in bag; seal securely. Turn bag to distribute marinade. Place in a shallow baking dish. Marinate in refrigerator 4 to 6 hours, turning occasionally.
2Remove tenderloin, reserving marinade. Pat tenderloin dry with paper towels. Melt butter in a large heavy skillet. Add tenderloin and brown on all sides. Add marinade; cover and simmer 10 minutes. Remove tenderloin, transferring marinade to glass measuring cup. Place tenderloin in shallow baking dish and place in freezer for 30 minutes.
3Meanwhile, preheat oven to 425 degrees F. In a mixing bowl, stir together flour and salt. Cut in shortening with a pastry blender till crumbs are the size of peas. In a small bowl, beat together egg yolk and 3 tablespoons of the cold water. Add the egg yolk mixture to the flour mixture, a little at a time, mixing with a fork after each addition. Add 1 to 2 tablespoons more cold water to the flour mixture, 1 tablespoon at a time, mixing with a fork till dough clings together. Form dough into a ball. Flatten dough; place between 2 sheets of waxed paper. Roll to a 1/8-inch-thick rectangle. Remove top sheet of waxed paper. Trim to a rectangle about 2 inches longer and 6 inches wider than tenderloin. Remove tenderloin from freezer and pat dry with paper towels. Place on pastry. Trim edges and of pastry. Fold in ends of pastry. Using the bottom sheet of waxed paper to guide pastry, wrap pastry around tenderloin, trimming to leave about 1/2-inch overlap. Seal edges with cold water.
4Line a baking sheet with foil; spray with nonstick coating. Transfer pastry-wrapped tenderloin to baking sheet. In a small bowl, beat together egg white and the 1 tablespoon water until foamy. Brush pastry with egg white mixture. Bake in preheated oven for 25-30 minutes, or till golden brown and until internal temperature on a thermometer reads 145 degrees Fahrenheit, followed by a 3-minute rest time. Remove pastry-wrapped tenderloin from oven; let stand 5 minutes.
5Meanwhile for sauce, measure the reserved marinade and add cold water to equal 1 cup. Transfer to a small saucepan. Stir in cornstarch, mixing well. Cook and stir over medium heat till slightly thickened and bubbly.
6To serve, slice tenderloin with sharp bread knife. Serve with sauce.