Step 1: Preheat your oven to 375°F (190°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.
Step 2: In a large bowl, whisk together the all-purpose flour, 1/2 cup (100g) granulated sugar, light brown sugar, baking powder, and salt.
Step 3: Add the cold, cubed unsalted butter to the dry ingredients. Use a pastry blender, your fingertips, or a food processor to cut the butter into the flour mixture until it resembles coarse crumbs with some pea-sized pieces remaining.
Step 4: Press about two-thirds of the crumble mixture evenly into the bottom of the prepared baking pan. Compact it firmly with the back of a spoon or your hands.
Step 5: Bake the crust for 12-15 minutes, or until lightly golden brown around the edges.
Step 6: While the crust is pre-baking, prepare the strawberry filling. In a medium bowl, gently toss the sliced fresh strawberries with the remaining 1/2 cup (100g) granulated sugar, cornstarch (or flour), and fresh lemon juice.
Step 7: Once the crust is lightly golden, carefully remove the pan from the oven. Spread the strawberry mixture evenly over the warm, pre-baked crust.
Step 8: Crumble the remaining one-third of the flour-butter mixture evenly over the strawberry filling.
Step 9: Return the pan to the oven and bake for an additional 30-40 minutes, or until the crumble topping is golden brown and the strawberry filling is bubbly and tender.
Step 10: Allow the bars to cool completely in the pan on a wire rack before attempting to cut them. Once cool, lift them out using the parchment paper overhang and cut into squares. Serve and enjoy!