Preheat the oven to 350°F (175°C) and line an 8x8-inch baking dish with parchment paper.
In a large mixing bowl, combine the all-purpose flour, rolled oats, brown sugar, melted butter, vanilla extract, salt, and ground cinnamon. Stir until it forms a crumbly texture.
Reserve 1/2 cup of the crumble mixture for topping. Press the remaining mixture into the bottom of the prepared baking dish to form a crust.
In a separate bowl, toss the mixed berries with honey (or maple syrup) and cornstarch. Spread the berry mixture evenly over the crust.
Sprinkle the reserved crumble mixture over the berry layer, ensuring the berries are completely covered.
Bake for 25-30 minutes, or until the topping is golden brown and the berries are bubbling.
Allow the crumble bars to cool in the pan for about 10-15 minutes, then lift them out using the parchment overhang and transfer to a wire rack to cool completely.
Once cooled, cut into squares or rectangles and enjoy.
Notes
Serve the crumble bars neatly arranged on a platter dusted with powdered sugar or pair with a scoop of vanilla ice cream.