Step 1: Preheat your oven to 375°F (190°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides to easily lift the bars out later. Lightly grease the parchment paper.
Step 2: In a large bowl, whisk together 1 ½ cups all-purpose flour, ½ cup granulated sugar, and ½ teaspoon salt. Add ½ cup (1 stick) cold unsalted butter, cut into small pieces. Using a pastry blender or your fingertips, cut the butter into the flour mixture until it resembles coarse crumbs.
Step 3: Press the mixture evenly into the bottom of the prepared baking pan. Bake for 12-15 minutes, or until lightly golden brown. This pre-baking step creates a crisp crust.
Step 4: While the crust bakes, prepare the rhubarb filling. In a medium bowl, combine 6 cups fresh rhubarb, chopped into ½-inch pieces, ⅔ cup granulated sugar, 2 tablespoons cornstarch, and 1 teaspoon vanilla extract. Toss gently to coat the rhubarb evenly.
Step 5: In another bowl, prepare the crumble topping. Combine 1 cup rolled oats, ½ cup all-purpose flour, ½ cup packed light brown sugar, and ½ teaspoon ground cinnamon. Add ½ cup (1 stick) cold unsalted butter, cut into small pieces. Use a pastry blender or your fingertips to cut the butter into the dry ingredients until coarse crumbs form.
Step 6: Once the crust is lightly golden, remove it from the oven. Spread the rhubarb filling evenly over the warm crust. Crumble the topping mixture over the rhubarb filling, distributing it as evenly as possible.
Step 7: Return the pan to the oven and bake for 35-45 minutes, or until the topping is golden brown and the rhubarb filling is bubbly and tender when pierced with a knife.
Step 8: Allow the bars to cool completely in the pan on a wire rack before cutting into squares and serving. This is crucial for the bars to set properly and hold their shape.