Heat oven to 350°

In large bowl , whisk together flours, cereal topping, sugar, baking soda and lemon peel.  With a pastry blender, cut in chilled butter until mixture resembles coarse crumbs.  Add egg (and oil if substituting) and cut together again.  Reserve 2 cups of flour mixture for "crisp" topping.  Press remaining flour mixture evenly into bottom of 15 1/2 x 10 1/2-inch brownie pan to form crust.  Bake 8-10 minutes.

In saucepan, combine filling ingredients. Cook over medium heat until rhubarb breaks down and juices thicken; stir frequently, to avoid burning.  Spread filling over crust. Sprinkle reserved crumb mixture over fruit filling.  Bake for another 15 minutes, or until crisp is golden brown.  Cool before cutting and serving.