Preheat oven to 375°. In a medium bowl, combine rhubarb, sugar, flour, and cinnamon. Evenly spread mixture in an 8- by 8-in. baking dish. Scatter butter over rhubarb, then cover with granola.
Bake until rhubarb is tender, 15 to 20 minutes. Serve warm with whipped cream.
Note: Nutritional analysis is per serving.