Preheat your oven to 375°F (190°C). Arrange the rhubarb pieces in an even layer in a buttered baking dish. In a small bowl, mix the sugar and cornstarch, then sprinkle evenly over the rhubarb. Bake for 10 minutes, just starting to soften the rhubarb.
4 cups rhubarb stalks, 0.5 cup granulated sugar, 1 tablespoon cornstarch
Create the crumble topping by combining oats, flour, and brown sugar in a mixing bowl. Add the cubed butter and use your fingers or a pastry cutter to work the butter into the dry ingredients until the mixture resembles coarse crumbs with visible pea-sized pieces.
1 cup old-fashioned rolled oats, 0.75 cup all-purpose flour, 0.5 cup brown sugar, 0.5 cup unsalted butter
Remove the partially baked rhubarb from the oven and evenly sprinkle the crumble mixture over the top, covering all the fruit.
Return the dish to the oven and bake for an additional 30 minutes, or until the topping is golden brown and the filling is bubbling around the edges. Look for bubbling liquid and a crisp, browned topping as doneness indicators.
Remove from oven and let cool slightly before serving. The crisp should have a crunchy topping with tender, tart rhubarb underneath.
