Preheat your oven to 350°F. While it heats, hull and slice the strawberries into 1/4-inch rounds, then trim the rhubarb and cut it into 1/2-inch pieces. Sift the corn starch into a small bowl to remove any lumps, which will help create a smooth, evenly thickened filling. Line a 9x13 inch baking dish with parchment paper for easy removal and cleanup. Having all your fruit prepped and ready before you start the crust ensures a smooth workflow and prevents the filling from oxidizing.

In a large bowl, combine the brown sugar, oats, and whole wheat flour. Cut the cold butter into small cubes and add it to the dry mixture. Using your fingers or a pastry cutter, work the butter into the dry ingredients until the mixture resembles coarse breadcrumbs with pea-sized pieces of butter throughout. I like to keep the butter pieces visible rather than fully incorporated—this creates those beautiful crispy, buttery clusters in the finished bars.

Reserve 1 cup of the oat mixture in a separate bowl for topping. Press the remaining oat mixture firmly and evenly into the prepared baking dish, creating an even layer across the bottom. Bake this crust for 10-15 minutes until it's set and lightly golden—this par-baking step helps prevent a soggy bottom and gives the crust structure to support the juicy fruit filling.

While the crust bakes, combine the sliced strawberries, rhubarb pieces, sifted corn starch, and sugar in a bowl. Gently fold the ingredients together until the fruit is evenly coated with the corn starch and sugar mixture. The sifted corn starch will distribute evenly and thicken the fruit juices as the bars bake, creating a jammy filling rather than a runny one.

Remove the pre-baked crust from the oven and spread the strawberry-rhubarb filling evenly over it. Sprinkle the reserved 1 cup of oat mixture over the top, breaking it into irregular clusters as you distribute it—these clusters will become the crispy topping. Return the baking dish to the oven and bake for 40-45 minutes until the fruit is bubbling around the edges and the topping is golden brown.

Remove the baking dish from the oven and let the bars cool in the pan for at least 30-45 minutes before cutting. I find that cooling them completely gives cleaner cuts and allows the filling to set up enough so the bars hold their shape. Once cooled, lift the bars out using the parchment paper overhang and cut into squares using a sharp knife.