Preheat the oven to 350F degrees.
Line a 9x13 inch pan with parchment paper leaving an overhang around all the edges, or line with aluminum foil & lightly grease.
In a large bowl stir together the flour, cinnamon, oats, & sugars
Stir in the melted butter and egg. It should look like wet sand.
Reserve about 1 ½ cups of the mixture for the topping. Then press the rest (about ⅔ of the batter) into the bottom of the prepared pan.
In a large bowl toss together the berries, sugar, lemon juice & cornstarch so that the berries are evenly covered.
Spoon the berry mixture over top of the base.
Crumble the remaining oatmeal mixture over top squeezing it in your palms so that large crumbles form.
Bake in the preheated oven for 45-50 minutes, or until the top is golden brown and you can see the berry mixture bubbling through.
Cool the pan on a wire rack at room temperature for 30 minutes, then move to the fridge to continue cooling (this can take up to 4 hours). before
Once the pan is completely cooled, slice into bars. Store in an airtight container in the fridge for up to 4 days.