Step 1: Preheat your oven to 375°F (190°C) and line a 9×13 inch baking pan with parchment paper, leaving an overhang on the sides. You can lightly grease the pan first to help the parchment stick.
Step 2: Prepare the crust. In a large bowl, whisk together 1 1/2 cups all-purpose flour, 1/2 cup granulated sugar, and 1/4 teaspoon salt. Cut in 1/2 cup cold unsalted butter using a pastry blender, two knives, or your fingertips until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.
Step 3: Press the crust into the pan. Evenly press the crumb mixture firmly into the bottom of the prepared baking pan. Bake for 15-18 minutes, or until lightly golden. Set aside to cool slightly.
Step 4: Prepare the rhubarb filling. In a separate medium bowl, combine the chopped rhubarb, 1 cup granulated sugar, and 1/4 cup all-purpose flour. Stir gently until the rhubarb is evenly coated.
Step 5: Prepare the crumble topping. In the same bowl you used for the crust, combine 1 cup all-purpose flour, 1 cup rolled oats, 1/2 cup packed light brown sugar, and 1/4 teaspoon salt. Cut in 1/2 cup cold unsalted butter until coarse crumbs form.
Step 6: Assemble the bars. Spread the rhubarb filling evenly over the partially baked crust. Then, sprinkle the crumble topping evenly over the rhubarb layer. Do not press the topping down.
Step 7: Bake the rhubarb crumble bars. Place the pan back into the preheated oven and bake for 45-55 minutes, or until the topping is golden brown and the rhubarb filling is bubbly and tender.
Step 8: Cool completely and serve. Allow the bars to cool completely in the pan on a wire rack before lifting them out using the parchment paper overhang and cutting them into 16 squares. Cooling thoroughly ensures the filling sets. Enjoy!