Preheat oven to 350F. Line an 8-inch square pan with aluminum foil and spray with cooking spray; set aside. This step is crucial for easy cleanup and removal.

In a large, microwave-safe bowl, melt the unsalted butter for about 1 minute on high power.

Add the flour, oats, both sugars, and salt to the melted butter. Stir to combine until the mixture is dry and sandy with some larger, well-formed crumble pieces.

Set aside 1 heaping cup of the mixture to be sprinkled on later as the crumble topping.

Transfer the remaining mixture to the prepared pan. Using a spatula or your fingers, hard-pack the mixture to create an even, smooth, flat crust; set aside.

In a large mixing bowl (the same one used for crust and crumble is fine), add the coarsely chopped fresh strawberries, granulated sugar, lemon juice, and cornstarch. Toss to combine. Don't worry if the sugar hasn’t dissolved fully; it will liquify while baking.

Evenly distribute the strawberry mixture over the crust.

Evenly sprinkle the reserved heaping 1 cup crumble topping mixture over the strawberries. For larger crumble pieces, gently squeeze the mixture in your palm before sprinkling.

Bake for about 50 minutes, or until the edges are set and the center has just set. The crumble topping should appear set and very pale golden, and the strawberry filling mixture will be bubbling along the edges. Note that baking time may vary if strawberries are very juicy; watch your bars, not just the clock.

Place the pan on a wire rack and allow the bars to cool for at least 1 hour before slicing and serving. Slicing too early will result in messy bars that may fall apart.