Preheat your oven to 350°F. Spray an 8×8-inch baking pan with cooking spray, then line it with parchment paper. Allow the parchment paper to overhang on each side as it makes it easy to lift our baked strawberry bars. Set aside.
Combine all the crust ingredients into a large mixing bowl.
Blend everything until it becomes saturated and collects together like a dough.
Transfer the prepared dough to the parchment paper-lined baking pan. Press the shortbread mixture firmly into the bottom of the pan. Place this pan into the preheated oven and partially bake the dough (for about 15 minutes) until it firms up and feels dry on the top.
Remove from oven and set it on the counter to cool completely.
Mix all the ingredients for the crumble topping in a bowl until it’s well blended. Set aside.
While the base is baking, combine strawberries, lemon zest, cornstarch, and sugar in a bowl.
Toss to combine.
Spread the strawberry topping on the cooled shortbread crust in a single layer. Cover your filling with the crumble topping.
Evenly distribute the crumble topping over the strawberry layer.
Bake the crumble bars for about 35-45 minutes or until the crumble topping has attained a slightly brown color.
Remove the bars from the oven and set them on the county to cool. Once cooled and firmed, carefully lift the parchment paper to slide the strawberry bars out from the pan. Using a sharp knife, cut into squares and enjoy!