Step 1: Preheat your oven to 375°F (190°C). Lightly grease an 8x8 inch baking dish or a similar sized oven-safe dish.
Step 2: In a large bowl, combine the chopped rhubarb with 1/2 cup granulated sugar, 2 tablespoons all-purpose flour (or cornstarch), 1 tablespoon lemon juice, and 1/2 teaspoon ground cinnamon. Toss gently until the rhubarb pieces are evenly coated.
Step 3: Pour the rhubarb mixture into your prepared baking dish. Spread it out evenly.
Step 4: In a separate medium bowl, combine the rolled oats, 1/2 cup all-purpose flour, 1/4 cup light brown sugar, 2 tablespoons granulated sugar, 1/4 teaspoon ground cinnamon, and a pinch of salt. Mix well with a fork until thoroughly 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 coarse crumbs form. You want some pea-sized pieces of butter remaining.
Step 6: Sprinkle the crumble topping evenly over the rhubarb filling in the baking dish.
Step 7: Place the baking dish on a baking sheet (to catch any potential drips) 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 fork.
Step 8: Remove from the oven and let the crumble cool for at least 15-20 minutes before serving. This allows the filling to set slightly.