Preheat oven to 375° F. Grease a 9x13-inch baking pan.
TO MAKE THE FILLING:
In a large mixing bowl, combine ingredients and stir until rhubarb is well coated.
Transfer mixture to prepared pan and spread evenly.
TO MAKE THE TOPPING:
In a separate mixing bowl, whisk together the gluten free flour blend, brown sugar, oats, and cinnamon until combined.
Add the butter and combine with a pastry blender, a fork, or your fingers until the mixture forms crumbs.
Sprinkle the mixture on top of the filling.
Bake 35 to 40 minutes, or until the topping is browned.
Serve warm or at room temperature.
Cut the rhubarb into equal size pieces. Equal size pieces will bake more evenly.
Verify your oats are certified gluten free. Although rolled oats are naturally gluten free, there may be cross-contamination at the facility where they’re processed.
Use cold butter to make the topping. The cold butter forms bigger crumbs than melted butter. I take my butter straight from the refrigerator and cut it into cubes before making the filling. This allows the butter to soften just slightly so I can use my fingers to combine it with the other topping ingredients.
Allow the crumble to cool for 15 minutes to allow the filling to thicken before serving.