Preheat oven to 375°F/190°C. Butter a 9x13-in. cast iron baking pan.
In a large bowl, combine all fruit filling ingredients. Pour into prepared pan.
In another large bowl, combine topping ingredients. Make sure the butter is evenly distributed; spread evenly on top of the rhubarb mixture.
Bake in oven for 45 - 60 minutes.
Optional: Top with a scoop of vanilla ice cream.
Recipe was created for our Calphalon Enamel Cast Iron Rectangular Baker.