Preheat oven to 350°. Line a 9-inch square baking pan with aluminum foil, allowing edges to extend over sides of pan. Spray with baking spray with flour. Set aside.
In a large bowl, combine flour, oats, brown sugar, and ½ teaspoon salt. Using a pastry cutter, cut in butter until mixture appears crumbly. Remove 1 cup oat mixture; set aside. Pat remaining oat mixture into the bottom of prepared pan. Bake for 10 minutes. Let cool on a wire rack for 30 minutes.
In a large bowl, whisk together pumpkin, condensed milk, pumpkin pie spice, lemon zest, eggs, and remaining ½ teaspoon salt. Pour over cooled crust. Sprinkle with reserved 1 cup oat mixture. Bake until oat mixture is golden brown and filling is set, about 45 minutes. Let cool completely on a wire rack. Refrigerate until chilled, about 4 hours. Using edges of foil, lift from pan. Cut into bars, using a serrated knife.
Recipe by Southern Lady Magazine at https://southernladymagazine.com/pumpkin-streusel-pie-bars/