Preheat oven to 350. Combine 1 ½ cups flour with the powdered sugar. Cut butter into the flour mixture with knife or pastry blender until it is well incorporated.
Transfer to 9x13 pan and pat evenly to cover bottom. Bake shell at 350 for 15 minutes. Set aside.
Meanwhile combine remaining ingredients, except rhubarb, and mix thoroughly. Stir in rhubarb and spread mixture over top of baked crust. Bake at 350 for 35-40 minutes.
When finished, the top layer should be crunchy and/or firm to the touch. It will also brown slightly. This dish is best when served immediately after it has cooled. To serve, top with whipping cream or milk.