1. Preheat oven to 400'F. Slice the rhubarb stalks on the diagonal into 3/4-inch-thick pieces. Place in a 9 by 13 inch baking dish; toss with 1 cup of the sugar & 1/4 cup of the flour.
2. In a food processor, pulse the remaining 1/2 cup of flour with the butter until the clumps are pea-size. Add remaining 1/2 cup of sugar, the rolled oats & cinnamon; pulse to combine. Sprinkle over the rhubarb.
3. Bake until the rhubarb is tender & the topping is golden - about 35mins. Serve with icecream, if desired.