Preheat the oven to 375°F (190°C) and lightly grease an 8x8 inch baking dish.
Make the Filling: In a medium bowl, combine rhubarb and granulated sugar. Toss to coat evenly, then spread in the prepared baking dish.
Make the Topping: In a separate bowl, whisk together oats, flour, brown sugar, and cinnamon. Pour melted butter over the mixture and stir until crumbly.
Assemble: Sprinkle the oat topping evenly over the rhubarb filling.
Bake: Place in oven and bake for 35-40 minutes, until filling is bubbly and topping is golden brown.
Cool and Serve: Let cool for at least 10 minutes before serving. Enjoy warm, alone, or with ice cream.
Notes
Add a pinch of ground ginger or nutmeg to the topping for a flavor twist. Increase sugar in filling if rhubarb is very tart. Works with frozen rhubarb; add 5-10 minutes to baking time if using.
Keyword easy dessert, fruit crumble, rhubarb crisp, summer dessert