01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper and lightly grease the paper.
02 - In a large mixing bowl, blend rolled oats, all-purpose flour, brown sugar, granulated sugar, baking powder, and salt.
03 - Pour melted unsalted butter into the oat mixture and mix until the ingredients form a crumbly texture.
04 - Reserve 1 cup of crumble mixture for topping. Firmly press the remainder into the base of the prepared pan to create an even crust.
05 - In a separate bowl, toss diced strawberries with lemon juice, cornstarch, and granulated sugar until the fruit is evenly coated.
06 - Evenly distribute the strawberry mixture over the crust.
07 - Scatter the reserved oat mixture evenly over the strawberries to form a crumbly top layer.
08 - Bake for 35 to 38 minutes, or until the topping is golden and the fruit filling is bubbling.
09 - Allow to cool completely in the pan. Use the parchment paper to lift out, then cut into 12 bars.