01 - Preheat oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper and lightly grease.
02 - In a large bowl, combine rolled oats, all-purpose flour, light brown sugar, granulated sugar, baking powder, and salt. Stir to blend. Pour in melted butter and mix until the mixture forms a crumbly texture.
03 - Reserve 1 cup of the oat mixture for the topping. Firmly and evenly press the remaining mixture into the prepared baking pan to create the base crust.
04 - In another bowl, mix diced strawberries with lemon juice, cornstarch, and granulated sugar. Gently stir the mixture until the strawberries are evenly coated.
05 - Evenly spread the strawberry filling over the pressed crust.
06 - Evenly distribute the reserved oat crumble over the strawberry layer.
07 - Bake in the preheated oven for 35 to 38 minutes, until the crumble topping is golden and the strawberry filling is bubbling at the edges.
08 - Let the bars cool completely in the pan. Lift out using the parchment paper and cut into 12 even bars.