Preheat the Oven: Set your oven to 350°F (175°C). No need to grease your 9×13-inch baking pan — the buttery crust ensures easy release.
Prepare the Crust: In a medium bowl, combine the flour, oats, brown sugar, and butter. Use a pastry cutter or your fingers to blend until the mixture resembles coarse crumbs. Press evenly into the bottom of the baking pan, making sure it’s well packed for a sturdy base.
Par-Bake the Crust: Bake the crust for 15 minutes, just until lightly golden and set. Remove from the oven and set aside while you prepare the filling.
Make the Filling: In a large bowl, whisk together the pumpkin, evaporated milk, eggs, pumpkin pie spice, salt, and brown sugar until smooth and well combined. The mixture should have the consistency of pumpkin pie batter — silky and pourable.
Assemble and Bake: Pour the pumpkin mixture evenly over the partially baked crust. Return to the oven and bake for another 20 minutes, or until the filling is beginning to set but still slightly soft in the center.
