Step 1: Preheat your oven to 375°F (190°C) and lightly grease an 8×8 inch baking dish.
Step 2: Prepare the rhubarb filling. In a large bowl, combine the chopped rhubarb, granulated sugar, 1/4 cup light brown sugar, cornstarch, and lemon juice. Toss gently until the rhubarb is evenly coated.
Step 3: Pour the rhubarb mixture into your prepared baking dish, spreading it evenly. Set aside.
Step 4: To make the crumble topping, in a separate medium bowl, whisk together the all-purpose flour, rolled oats, 1/2 cup light brown sugar, 1/4 cup granulated sugar, ground cinnamon, and salt. Make sure all dry ingredients are well combined.
Step 5: Add the cold, unsalted butter, cut into small cubes, to the dry ingredients. Using your fingertips, a pastry blender, or two knives, cut the butter into the flour mixture until it resembles coarse crumbs with some pea-sized pieces remaining.
Step 6: Sprinkle the crumble topping evenly over the rhubarb filling in the baking dish. Do not press it down too firmly.
Step 7: Place the baking dish on a baking sheet (to catch any potential overflow) and bake for 35-45 minutes, or until the topping is golden brown and the rhubarb filling is bubbling around the edges.
Step 8: Once baked, carefully remove the crumble from the oven and let it cool on a wire rack for at least 15-20 minutes before serving. This allows the filling to set slightly.