01 -
Preheat your oven to 350°F (175°C) and lightly grease a 9×9-inch or 11×7-inch baking pan.
02 -
Mix together the flour, oatmeal, brown sugar, melted butter, and cinnamon in a medium bowl until crumbly.
03 -
Press half of the crumb mixture evenly into the prepared baking pan.
04 -
Spread the diced rhubarb evenly over the pressed crumb layer.
05 -
In a small saucepan over low heat, combine the white sugar, water, and cornstarch. Stir continuously and bring to a boil, cooking until the syrup thickens and becomes clear.
06 -
Pour the hot syrup evenly over the rhubarb layer in the pan.
07 -
Sprinkle the remaining crumb mixture over the top to cover.
08 -
Bake for 1 hour, or until the topping is golden brown and the rhubarb is bubbling.
09 -
Allow the dessert to cool slightly before serving.