Preheat your oven to 350°F (180°C) and prepare an 8x8 inch baking dish by lining it with parchment paper.

In a large mixing bowl, combine old fashioned rolled oats, all-purpose flour, and brown sugar. Pour in melted unsalted butter and mix until it resembles a crumbly dough.

Reserve 1 cup of the crumb mixture for topping. Press the remaining mixture evenly into the bottom of the prepared baking dish to form a crust.

In a separate bowl, gently combine fresh or thawed raspberries, cornstarch, and a sprinkle of sugar until well coated. Spread this mixture evenly over the crust.

Crumble the reserved oat mixture generously over the raspberry layer.

Bake for about 30 minutes, or until the topping is golden brown and the filling bubbles around the edges.

Let it cool at room temperature for 10 to 15 minutes, then transfer to the refrigerator to chill for at least 1 hour before cutting.