Preheat oven to 350 degrees F. Line an 8x8”-inch baking dish with parchment paper,leaving some parchment going up the sides about 1 inch so you can easily lift out of the pan later.
In a medium sized bowl, combine all the ingredients for the rhubarb filling. Toss with a spatula to coat everything. Set aside while preparing the crumble.
Ina food processor, combine all the crumble ingredients. Mix until the butter has completely mixed in and the mixture appears in small crumbles. Pour half of the crumble mixture into the baking dish and press it down evenly throughout the pan.
Evenly distribute the rhubarb filling over the pressed crumble. Top with the remainder of the crumble topping, and press down lightly until all the crumble is used.
Bake for 1 hour. Cool at room temperature for 2+ hoursbefore removing from pan and cutting. Cut into 9 squares. Enjoy!