- Preheat oven to 325F. Butter 9×9 pan or Pyrex baking dish.
- Mix all ingredients except rhubarb until crumbly. Reserve 1 cup for topping.
- Add rhubarb to mixture and stir to combine. Pour into greased pan/dish and top with the reserved crumble. Sprinkle cinnamon as desired.
- Bake for 1 hour.
Optional: Add a few drops of red food coloring to rhubarb and toss well before adding to prepared crumble, if desired.