Preheat the oven to 350 °F, and spray a 9x13 baking dish with non-stick cooking spray.
Place diced rhubarb in the bottom of the baking dish.
In a medium-sized saucepan, add the water, sugar, and cornstarch. Whisk together until there are no clumps of cornstarch. Heat over medium heat, stirring occasionally until thickened. Stir in the vanilla and stir to combine.
Pour the cornstarch mixture evenly over the rhubarb.
In a separate bowl, combine the flour, oatmeal, brown sugar, and cinnamon together. Mix until combined. Pour in the melted butter and stir until everything has a coating of butter and there are no large clumps. Sprinkle over the rhubarb.
Bake uncovered for 35 to 45 minutes.
Notes
May be served warm or cold. Store baked crunch dessert covered in the refrigerator.
This recipe was calculated using the exact brands and measurements I used to make this recipe. If you are following a strict diet please note changing anything will cause the nutritional info to change. My calculations are intended as a guide only.