Preheat the oven to 350ºF degrees. Line an 8x8-inch baking pan with parchment paper leaving an overhang around the edges. Or line with aluminum foil leaving an overhang around the edges and grease with nonstick cooking spray.

In a medium bowl whisk together the flour, oats, granulated sugar, brown sugar, cinnamon, and salt. Gently stir in the melted butter and egg.

Remove about 1 cup (not packed) of the mixture for the topping; set aside.

Press the remaining mixture into the bottom of the prepared pan.

In a medium bowl stir together the berries, sugar, lemon juice and cornstarch until the berries are coated.

Spoon the raspberry mixture over top of the oatmeal base. It's okay if there are empty spaces on the base, as the raspberries bake they will spread.

Crumble the reserved oat mixture over top of the fruit layer. I like to form some big and some small crumbles and make sure all of the raspberries are topped.

Bake in the preheated oven for about 50-55 minutes or until you see the berries bubbling and the top crumble turning golden brown.

Cool the bars fully (while keeping them in the pan) until they're room temperature (at least 3 hours) before slicing.

To slice the bars, lift them out of the pan using the overhang of the parchment paper/aluminum foil. Place on a cutting board and slice with a large, sharp knife.

Bars can be served cold from the fridge, room temperature, or warmed up in the microwave. They're so good alongside a scoop of vanilla ice cream.