In a large mixing bowl, beat egg. Add the sugar, flour and vanilla; mix well. Gently fold in rhubarb and strawberries. Pour into pastry shell.
For topping, combine flour, brown sugar and oats in a small bowl; cut in butter until crumbly. Sprinkle over fruit. Bake at 400 degrees F for 10 minutes. Reduce heat to 350 degrees F bake for 35 minutes or until golden brown and bubbly. Cool on a wire rack.
NOTE: Place on bakers rack on a foil lined cookie sheet as the pie will bubble over! Once cooled, place in refrigerator until set for nice slicing! Keep pie refrigerated. A dollop of ice cream along side a heated up slice of pie is divine!