To celebrate the homey, garden-to-oven nature of this simple cake, serve it warm from the oven.
In a medium bowl stir together the flour, baking powder, baking soda, cinnamon, and salt; set aside. In a large mixing bowl combine the 2/3 cup granulated sugar, brown sugar, oil, and egg; beat till combined. Add the oat mixture and beat well. Add the flour mixture to the oat mixture, beating just till combined.
Carefully pour the cake batter atop the rhubarb mixture. Bake in a 350° oven about 50 minutes or till a toothpick inserted near the center comes out clean. Cool on a wire rack for 5 minutes. Run a knife along sides of pan to loosen cake; invert onto a serving plate. Serve warm. Makes 9 servings.
Note: If fresh rhubarb is unavailable, measure 2 cups frozen sliced rhubarb; thaw slightly and chop. Return to bowl and thaw completely before using.
Rhubarb tips: When choosing rhubarb, select firm and tender stalks; avoid those that are very thick or wilted. To preserve its crisp texture, wrap raw rhubarb tightly in plastic wrap and refrigerate for up to 1 week. One pound of raw rhubarb yields 3 cups sliced fruit.