Preheat the oven to 350°F (175°C). Line an 8-inch square pan with aluminum foil and spray with cooking spray; set aside. Lining the pan helps with easy cleanup and is highly recommended.
In a large microwave-safe bowl, melt the butter, about 1 minute on high power.
Add the flour, oats, granulated sugar, brown sugar, and salt to the melted butter. Stir to combine. The mixture will be dry and sandy with some larger crumble pieces.
Set aside 1 heaping cup of the mixture to use as the crumble topping.
Press the remaining mixture into the prepared pan, using a spatula or your fingers to create an even, flat crust. Set aside.
In the same mixing bowl used for the crust, combine the chopped strawberries, sugar, lemon juice, and cornstarch. Toss to combine.
Evenly distribute the strawberry mixture over the crust in the pan.
Sprinkle the reserved crumble mixture over the strawberry layer. Before sprinkling, squeeze the mixture in your hands to form larger crumble pieces.
Bake for about 50 minutes, or until the edges are set and the center has just set. The crumble topping should appear pale golden, and the strawberry filling will be bubbling along the edges. Depending on how juicy the strawberries are, the bars may take longer than 50-55 minutes. Watch the bars, not the clock, when determining doneness.
Remove from the oven and place the pan on a wire rack. Allow the bars to cool for at least 1 hour before slicing and serving. Slicing the bars too early will make them messy and prone to falling apart.