Step 1: Preheat your oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper or aluminum foil, leaving an overhang on the sides for easy lifting. Lightly grease the lining with cooking spray.
Step 2: In a large mixing bowl, combine the melted butter, all-purpose flour, old-fashioned oats, granulated sugar, light brown sugar, and salt. Stir with a spoon or your hands until the mixture is well combined and crumbly.
Step 3: Reserve about 1 cup of the oat crumble mixture for the topping. Press the remaining crumble mixture firmly and evenly into the bottom of the prepared baking pan to form the crust.
Step 4: In a separate medium bowl, gently toss the chopped strawberries with ¼ cup granulated sugar, lemon juice, and cornstarch until the strawberries are evenly coated. Pour this strawberry mixture over the pressed crust in the pan, spreading it to an even layer.
Step 5: Evenly sprinkle the reserved 1 cup of crumble mixture over the strawberry filling. You can gently press some of the crumbs together to form larger clusters for a more rustic topping.
Step 6: Bake for 50-60 minutes, or until the edges are golden brown and the strawberry filling is bubbly. The top crumble should also be lightly golden.
Step 7: Remove the pan from the oven and let the bars cool completely on a wire rack before lifting them out of the pan using the foil/parchment overhang. Once cooled, slice into squares and serve.