Preheat your oven to 375º and prepare a small baking dish (5"x7") with cooking spray; set aside. In a medium sized bowl add the rhubarb, flour, corn starch, sugar and cinnamon; toss to coat. Spread the rhubarb mixture evenly over the bottom of the baking pan and dot with the 1/2 Tbs butter.
In a small bowl combine the oats, sugars and flour, cut in the butter with your fingers until the mixture is crumbly.
Spread the topping mixture over the rhubarb and bake for 45 minutes - 1 hour, until bubbly and golden on top. Carefully remove from the oven to cool 8-10 minutes, serve warm.
Recipe by It Bakes Me Happy at http://itbakesmehappy.com/2013/02/rhubarb-crunch.html