In a 2-qt. microwave-safe dish, combine the rhubarb, sugar and flour. Cover and microwave on high for 3 minutes; stir. Add pie filling.
In a small bowl, combine the flour, oats, brown sugar and cinnamon. Cut in butter until mixture is crumbly; sprinkle over filling. Cover and cook 4-5 minutes longer or until bubbly and rhubarb is tender. Serve warm.
Editor’s Notes: If using frozen rhubarb, measure rhubarb while still frozen, then thaw completely. Drain in a colander, but do not press liquid out.