Preheat the oven to 375 degrees.
Grease an 8 x 8-inch pan.
Add rhubarb, frozen mixed berries, white sugar, 1/3 cup flour, and cinnamon to a bowl.
Mix the ingredients together until all the rhubarb and berries are coated.
Dump the rhubarb mixture into the greased baking dish in an even layer.
Next add the ghee, coconut oil, 1 cup white flour, oats, brown sugar, and salt to a mixing bowl.
Mix the crumble ingredients together (you may need to use a fork to cut the coconut oil and ghee into the flour mixture).
Once the crumble topping is mixed & crumbly, scoop it onto the top of the rhubarb mixture in an even layer.
Place the pan in the oven (if you are worried the crumble may drip over the edge, you can place a 9 X 13-inch pan under the pan) and bake the crumble for 30-35 minutes or until the top begins to brown.
Remove the pan from the oven and allow the rhubarb berry crumble to cool slightly before consuming.
You can top your crumble with some dairy free ice cream for an added bonus!