Preheat your oven to 350°F (175°C) and grease a 9-inch square baking pan.
In a mixing bowl, toss the sliced strawberries with sugar and cornstarch until evenly coated. Set aside to let the juices release and thicken slightly.
In a separate bowl, combine oats, flour, and brown sugar. Mix well to distribute the dry ingredients evenly.
Add the cold, cubed butter to the dry mixture. Use a pastry cutter or your fingers to work the butter into the dry ingredients until the mixture resembles coarse crumbs.
Press half of the crumble mixture firmly into the prepared baking pan to form an even crust.
Pour the strawberry filling over the crust, spreading it out evenly with a spatula to cover the surface.
Sprinkle the remaining crumble mixture evenly over the strawberry layer, covering it completely.
Bake in the preheated oven for about 35 minutes, or until the crumble is golden brown and the filling is bubbling around the edges.
Remove from oven and let the bars cool in the pan for at least 15 minutes to set before cutting into squares.
Slice into rectangles and serve once cooled. Enjoy the warm, crunchy topping with the juicy, jammy strawberries inside.
