Preheat your oven to 350°F (175°C). Line an 8x8-inch baking dish with parchment paper, allowing some overhang for easy removal later.
In a mixing bowl, combine the flour, rolled oats, brown sugar, melted butter, vanilla extract, salt, and ground cinnamon. Mix until crumbly.
Reserve 1/2 cup of this mixture for the topping. Press the remaining mixture firmly into the bottom of the prepared baking dish to form a crust.
In another bowl, gently toss the mixed berries with the honey (or maple syrup) and cornstarch. Spread the berry mixture evenly over the crust in the pan.
Crumble the reserved oat mixture over the top of the berries, covering them fully.
Bake in the preheated oven for 25-30 minutes, or until the top is golden and the berries are bubbly.
Remove from the oven and allow to cool in the pan for about 10-15 minutes. Use the parchment overhang to lift the bars out of the dish and let them cool completely on a wire rack.
Once cooled, cut into squares or bars and serve.
Notes
Allow the bars to cool completely before cutting for best results.