Preheat your oven to 350°F (175°C). Grease or line a 9x9-inch baking pan with parchment paper.

Prepare the Fruit Filling: In a medium saucepan, combine the diced rhubarb and 2 tbsp of granulated sugar. Cook over medium heat, stirring occasionally, until the mixture thickens and the fruit breaks down slightly, about 10-12 minutes. Remove from heat and let cool.

Combine the cooled rhubarb with the strawberries, granulated sugar, cornstarch, lemon zest, and lemon juice.

Make the Crumble: In a large mixing bowl, combine the oats, flour, brown sugar, baking powder, and salt.

Add the cold cubed butter and use your fingers or a pastry cutter or (I like to mix it in my standup Kitchen-Aid mixer) until the mixture resembles coarse crumbs.

Assemble the Bars: Press about two-thirds of the crumble mixture firmly into the bottom of the prepared baking pan to form an even layer.

Spread the cooled fruit filling evenly over the crumble base.

Sprinkle the remaining crumble mixture evenly over the top of the fruit filling, allowing some of the fruit to peek through, I leave the topping loose, I don not press the top layer down.

Bake and Serve: Bake in the preheated oven for 35-40 minutes, or until the crumble topping is golden brown and the fruit filling is bubbling and delicious!

Remove from the oven and let cool completely in the pan on a wire rack.Once cooled, lift the bars out of the pan using the parchment paper and cut into squares or bars.