Combine flour, oats, brown sugar and butter in a mixer. Mix for a few minutes, until everything is crumbly. Press the mixture into the base of an ungreased 13x9-inch baking pan. Bake for 15 minutes.
While your crust is baking, combine sugar, can of pumpkin, evaporated milk, eggs, and pumpkin pie spice in your mixer and beat for about two minutes at medium speed.
Once the crust is finished baking, pour your mixture on top and return to the oven to bake for another 20 minutes.
Meantime, chop your pecans and combine them with 1/4 cup of brown sugar. Sprinkle the pecan topping over the filling and return to the oven to bake for another 20-25 minutes.
Cool and then cut into bars.
Enjoy with vanilla ice cream and/or whipped cream.
Once your bars have cooled completely stick the whole pan in the fridge for 30 minutes before cutting into squares. Store the squares in an airtight container in the fridge and take them out just before serving. You can also freeze the squares in an air-tight container in layers separated by parchment paper.