Preheat your oven to 350°F (175°C). Line an 8-inch square baking pan with parchment paper.

Melt the unsalted butter in a large mixing bowl. Add the all-purpose flour, old-fashioned oats, granulated sugar, light brown sugar, and salt. Stir until the mixture resembles wet sand.

Take 1 cup of the mixture and set it aside for the crumble topping. Press the remaining mixture firmly into the bottom of the lined baking pan.

In a medium bowl, combine the chopped strawberries, additional granulated sugar, lemon juice, and cornstarch. Mix gently until evenly coated.

Spoon the strawberry mixture over the pressed crust in the baking pan, spreading it evenly.

Sprinkle the reserved crumble topping evenly over the strawberry filling.

Bake in the preheated oven for 50 minutes or until the top is golden brown and the filling is bubbly.

Remove from the oven and let cool for at least 1 hour. Then lift the bars out using the parchment and slice into squares.