In a large bowl, stir together cracker crumbs, sugar, and melted butter until well combined. Firmly press mixture into bottom and up sides of a rimmed 15x10-inch baking sheet.
Bake until lightly browned, about 10 minutes. Let cool completely.
In a large bowl, whisk together condensed milk, eggs, lime zest and juice, vanilla, and salt until combined; let mixture stand for 10 minutes (mixture will begin to thicken). Spread into prepared crust.
Bake until just set, about 10 minutes. Let cool completely on a wire rack.
Loosely cover, and refrigerate until cold before serving, at least 4 hours, or up to 3 days. Just before serving, spread with whipped topping, and garnish with lime, if desired.
Recipe by Paula Deen Magazine at https://pauladeenmagazine.com/key-lime-slab-pie-recipe/