Preheat oven to 350 degrees.
Spray a 9 x 9-inch baking pan with non-stick spray. Set aside.
Add raspberries to a large bowl followed by sugar and cornstarch. Toss to combine, then add berries to the baking dish and distribute the sugar/cornstarch mixture that's left in the bowl over the fruit.
Make the topping by mixing the flour and brown sugar together in a medium bowl. Cut in the flour using a pastry blender, two knives or your fingers, until the butter is well distributed. Then pinch and squeeze the topping mixture with your fingers to make nice crumbs.
Sprinkle the crumbs over the berries.
Bake for 35-45 minutes or longer, until the middle of the crisp is bubbling.
Serve warm with a scoop of vanilla ice cream or sweetened whipped cream.
