Preheat your oven to 350°F and line a 9x13 inch baking dish with parchment paper, leaving a slight overhang for easy removal later. In a medium bowl, whisk together the flour, sugar, and salt for the crust. Pour in the melted and slightly cooled butter, stirring until the mixture resembles wet sand with small clumps throughout. Press this mixture firmly and evenly into the prepared baking dish, using the bottom of a measuring cup or your fingertips to create an even layer. Bake for 12-15 minutes until the crust is light golden and set but not deeply browned—it will continue to cook when the crumble bakes on top.

While the crust bakes, hull and slice the strawberries into 1/4-inch rounds into a medium bowl. Add the sugar, cornstarch, lemon zest, lemon juice, and vanilla extract, and gently fold everything together until the strawberries are evenly coated. Let this mixture sit for 5 minutes—this allows the cornstarch to begin absorbing the strawberry juices, which prevents a soggy filling.

In a separate bowl, whisk together the flour, brown sugar, cinnamon, and salt for the crumble. Pour in the melted butter and stir until the mixture forms small, irregular clumps—I prefer leaving some larger pieces mixed with finer crumbs because this creates better texture contrast between crispy and tender spots. Don't overmix; a slightly chunky texture is exactly what you want here.

Remove the pre-baked crust from the oven and carefully spread the strawberry filling evenly over it, along with all the juices that have accumulated. Working quickly so the crust doesn't cool too much, scatter the crumble topping from Step 3 evenly over the strawberries, breaking up any larger clumps as you go—don't press down on the crumble or it will become dense and cake-like. Return the bars to the oven and bake for 30-35 minutes until the crumble is golden brown and the strawberry filling is bubbling slightly at the edges.

Remove the bars from the oven and let them cool in the baking dish at room temperature for at least 1 hour—this cooling time is crucial as it allows the filling to set and makes cutting much cleaner. Once completely cooled, you can optionally chill the bars for another hour in the refrigerator if you prefer them firmer, though it's not necessary. Dust the top generously with powdered sugar just before cutting and serving.