Place the first 3 ingredients in a food processor, and pulse 2 to 3 times. Add margarine pieces, and pulse 10 times or until the mixture resembles coarse meal. With the processor on, slowly add ice water through food chute, processing just until combined (do not form a ball).
Gently press the flour mixture into a 4-inch circle on heavy-duty plastic wrap, and cover with additional plastic wrap. Roll the dough, still covered, into a 15 x 13-inch rectangle. Place dough in freezer 5 minutes or until plastic wrap can be easily removed. Remove 1 sheet of plastic wrap. Fit dough into a 2-quart baking dish coated with cooking spray, allowing dough to extend over edges of dish; remove top sheet of plastic wrap.
Combine peaches, 1/2 cup of brown sugar, 2 1/2 tablespoons flour, vanilla, and cinnamon in a large bowl; toss gently. Spoon peach mixture into prepared dish; fold edges of dough over peach mixture (it will only partially cover peaches). Sprinkle remaining 1/4 cup brown sugar over peach mixture; sprinkle with almonds.
Combine egg and water in a small bowl; stir well. Brush egg mixture over dough, and sprinkle with 1 tablespoon sugar. Bake at 375° for 45 minutes or until filling is bubbly and crust is lightly browned. Let stand 30 minutes before serving.