Easy Key Lime Pie

Ingredients

• 2 cups graham cracker crumbs
• 1/2 cup granulated sugar
• 1/2 cup (4 oz.) salted butter, melted, plus more for greasing dish
• 1 (14-oz.) can sweetened condensed milk
• 1/2 cup key lime juice
• 1 tablespoon lime zest (from 2 limes)
• 1 (8-oz.) container frozen whipped topping (such as Cool Whip), divided
• 1 lime, thinly sliced