Preparation

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  Preheat oven to 375 and grease a 9 x 13 tray. Use a cheese grater to grate the butter onto a plate and then place the grated butter in the freezer while you mix the rest of the ingredients. This works best with butter from the fridge, not room temperature or frozen butter.
  In a large bowl stir together the flour, graham cracker crumbs, ½ cup oats (do not add the 2 tablespoons yet), brown sugar, white sugar, lemon zest, and salt. Add the egg and egg white and mix them into the dry ingredients with a fork.
  Get the grated butter from the freezer and add it to the crumble mixture. Use your hands to rub the butter into the flour until it resembles crumbs the size of peas. Remove two cups of the crumble mixture to use as the topping and stir the 2 tablespoons of oats into this mixture.
  Press the remaining crumble mixture into the greased pan into an even layer.
  In a different bowl stir together the chopped rhubarb, strawberries, sugar, vanilla and corn starch. Spread on top of the layer of pressed crumble. Top with the reserved two cups of crumble mixture.
  Bake for 45 to 50 minutes until the top is golden and the filling is bubbling. Allow to cool completely before cutting into bars. While warm, the rhubarb flavor is a lot sharper and sour, it mellows and sweetens as it cools. These bars are excellent leftover. Cover with plastic wrap loosely to avoid soggy topping.