Heat oven to 400°F. Unfold pastry sheet; place in 9-inch square tart pan with removable bottom. Brush pastry with egg white. Bake 15 to 20 minutes or until browned.
Meanwhile, in 12-inch skillet, cook bacon over medium-high heat 8 to 11 minutes or until crisp; drain on paper towels. Crumble bacon; set aside. Reserve 1 tablespoon bacon drippings in skillet. Add leeks to drippings; cook over medium heat 5 to 7 minutes, stirring occasionally, until tender. Remove from heat. Stir in 1/2 cup of the cheese, the chopped thyme, salt and pepper.
Press pastry with back of spoon to flatten. Spoon leek mixture over pastry; sprinkle with crumbled bacon and remaining 1/2 cup cheese. Bake 5 to 7 minutes or until cheese is melted. Serve warm.