In a small saucepan, melt butter; stir in crumbs. Reserving 1 Tablespoon for garnish, pat crumbs on bottom of buttered 8 x 8 inch baking pan; set aside. In medium bowl, combine pie filling, sweetened condensed milk, and lemon juice; mix until smooth. Spread in prepared pan. Top with whipped topping and reserved crumbs. Freeze 3 hours. If desired, garnish with lemon slices before serving.