Adapted from Dorie Greenspan's recipe
Center a rack in the oven and preheat oven to 350 F.
Butter a 9-inch square baking pan (I used one 7x10 inch and one 5x7 inch baker).
Line a baking sheet with a Silpat or piece of parchment paper.
Step 1: Crisp Crust and Topping
1 cup (5 oz.) all-purpose flour
1 cup (7 oz.) packed light brown sugar
3/4 cup old-fashioned rolled oats
1/2 tsp. ground cinnamon
1/2 cup finely chopped walnuts
1 stick (8 Tablespoons) unsalted butter, cut into pieces and chilled
Put the flour, brown sugar, oats, cinnamon, and salt in the bowl of a stand mixer and mix on low speed until combined. Add the butter a few pieces at a time on medium speed until butter is mixed in and the mixture looks like wet sand (this can also been done by hand with a pastry blender). Add in the chopped nuts.
Spoon half of the mixture into the baking pan and pat it down to form a thick crust; set aside the remainder for the topping.