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Preheat the oven to 350 degrees F. Lightly grease an 8-inch square baking pan. You can also line the pan with parchment for easier removal after baking.
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In a food processor fitted with a knife blade, add the sugar, brown sugar, flour, oats, cinnamon, baking soda, and salt.
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Pulse one or two times to combine. Add the diced butter. Pulse until it resembles coarse crumbs.
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Transfer the crumb mixture into a large bowl. Stir in the walnuts.
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Set aside 1 1/2 cups of the crumb mixture in a bowl to be used for the topping.
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Pour the remaining crumb mixture into the baking pan, spread evenly, and firmly press down.
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Bake the crust for 15 minutes. Pour the filling over the baked crust and spread evenly. Sprinkle reserved crumb mixture on top.
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Return the pan to the oven and bake for 20 minutes or until set.
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Cool completely on a wire rack. Slice into bars or squares.