In large bowl, whisk together brown sugar, cornstarch, and cinnamon. Add peaches and lemon juice; toss to coat. Set aside.
Remove two pie crusts from pouches. On a lightly floured surface, unroll and stack crusts one on top of the other. Roll into a 17x12-inch rectangle. Fit crust into an ungreased 15x10x1-inch pan, pressing into the corners, allowing excess to overhang. Do not trim. Pierce the pastry all over with a fork. Spread the peach filling over the pastry and arrange evenly.
Roll out remaining two pie crusts in the same manner, and place on top of filling. Pinch the edges of bottom and top crusts together to seal, tuck under any extra pastry crust, as needed. Use a knife to cut vents across top of pie crust, or cut out decorative shapes in top crust before topping pie, or cut out strips and interweave to create a lattice top.
Evenly sprinkle 1 tablespoon of sugar over the top of the pie.
Bake for 35-40 minutes, until bubbly and golden brown. Remove from oven and set aside to cool completely before cutting into squares. Serve with vanilla ice cream, if desired.
Cook's Note: If you want to make your own pie crust, I recommend Papa's Pie Crust. (Double the recipe for this slab pie.)