Prep the Fruit Filling: Pit and chop fresh cherries, and slice rhubarb. In a bowl, combine cherries, rhubarb, granulated sugar, and cornstarch. Toss well and let sit for 10 minutes.

Make the Crust and Crumble: In a large bowl, mix rolled oats, all-purpose flour, brown sugar, and cinnamon. Melt unsalted butter and pour it into the dry ingredients, stirring until crumbly. Reserve 1 cup for topping.

Assemble the Bars: Preheat oven to 350°F (175°C) and grease a 9x9-inch pan. Press half of the crumble into the bottom. Spoon fruit filling over and sprinkle reserved crumble on top.

Bake to Perfection: Bake for 35-40 minutes until edges are golden brown and filling bubbles. Remove and cool on a wire rack.

Cool and Slice: Allow to cool completely, about an hour. Slice into squares for serving.