1. Preheat oven to 400°. Remove and discard root ends and dark green tops of leeks. Thinly slice leeks, and rinse thoroughly under cold running water to remove grit and sand.
2. Cook bacon in a large skillet over medium-high heat 8 to 11 minutes or until crisp; remove bacon, and drain on paper towels, reserving 1 Tbsp. drippings in skillet. Crumble bacon.
3. Sauté leeks in hot drippings over medium heat 5 to 7 minutes or until tender. Stir in 1/2 cup cheese, thyme, pepper, and salt.
4. Unfold pastry sheet; fit into a 9-inch square tart pan. Whisk egg white until light and frothy. Brush egg white onto pastry sheet.
5. Bake at 400° for 15 to 20 minutes or until browned. Remove from oven. Press pastry with back of a spoon to flatten. Top with leek mixture; sprinkle with crumbled bacon and remaining 1/2 cup cheese. Bake 5 to 7 minutes or until cheese is melted.
* Swiss cheese may be substituted.
** 1 tsp. dried thyme may be substituted.