In a stand mixer, combine the flour, oats, 1/2 cup brown sugar and butter. Beat on low speed for 2 minutes or until the mixture looks crumbly. Press the oat mixture on the bottom of an ungreased 13x9-inch (33cm x 23cm) baking pan.
Bake for 15 minutes.
In a large bowl, whisk together the granulated sugar, pumpkin, evaporated milk, eggs and pumpkin pie spice until just combined. Pour the pumpkin mixture over the crust.
Bake for 20 minutes.
Meanwhile, combine the pecans and the remaining brown sugar in a small bowl. Sprinkle pecan topping over filling. Continue baking for 20 minutes or until a toothpick inserted in the center comes out clean. Cool completely in the pan on a wire rack. Cut into bars.
Notes
Cover the baking pan with plastic wrap to store in the refrigerator. Chilling the dessert will also help make cleaner cuts.