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 into the bottom of an ungreased 13x9-inch (33 cm x 23 cm) 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 and bake for 20 minutes.
Meanwhile, combine the pecans and the remaining brown sugar in a small bowl. Sprinkle the pecan topping over the filling. Continue baking for an additional 20 minutes, or until a toothpick inserted in the center comes out clean. Cool completely in the pan on a wire rack, then 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.