Step 1: Preheat your oven to 375°F (190°C). Line a 9×13-inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.
Step 2: In a large bowl, combine the all-purpose flour, rolled oats, light brown sugar, baking powder, and salt. Whisk them together until well combined.
Step 3: Add the very cold, cubed unsalted butter to the dry ingredients. Use a pastry blender, your fingertips, or a food processor to cut the butter into the mixture until it resembles coarse crumbs with some pea-sized pieces remaining.
Step 4: Reserve about 1 cup of the crumble mixture for the topping. Press the remaining crumble mixture evenly into the bottom of the prepared baking pan, compacting it firmly with the back of a spoon or your hands.
Step 5: Bake the crust for 12-15 minutes, or until it is lightly golden brown. Remove from the oven.
Step 6: While the crust bakes, prepare the strawberry filling. In a medium bowl, gently toss the hulled and sliced fresh strawberries with the granulated sugar, cornstarch, and vanilla extract (and optional lemon juice). Ensure the strawberries are evenly coated.
Step 7: Carefully spread the prepared strawberry filling evenly over the warm, pre-baked crust in the baking pan.
Step 8: Evenly sprinkle the reserved crumble mixture over the strawberry layer.
Step 9: Return the pan to the oven and bake for another 35-45 minutes, or until the crumble topping is golden brown and the strawberry filling is visibly bubbly and tender. If the top browns too quickly, you can loosely tent with aluminum foil.
Step 10: Allow the bars to cool completely in the pan on a wire rack for at least 2-3 hours before slicing. This is crucial for the filling to set properly, ensuring clean cuts. Once cooled, use the parchment paper overhang to lift the entire slab out and slice into 12 bars.