Preheat the Oven: Preheat your oven to 400°F (200°C). Lightly butter a 2-quart casserole dish.

Make the Rhubarb Filling: In a large bowl, combine the sliced rhubarb, cornstarch, sugar, and cinnamon. Mix until the rhubarb is evenly coated. Spread the filling evenly in the prepared casserole dish.

Prepare the Crumble Topping: In another large bowl, combine the oats, flour, sugar, cinnamon, and salt. Add the cold, cubed butter and use your fingers or a pastry cutter to mix until the mixture resembles coarse crumbs.

Assemble and Bake: Evenly scatter the crumble topping over the rhubarb filling. Bake in the preheated oven for about 35 minutes, or until the topping is golden brown and the filling is bubbly.

Cool and Serve: Allow the rhubarb crisp to cool on the counter for at least 10 minutes before serving. Serve warm with a scoop of vanilla ice cream.