In a large bowl, beat egg. Beat in the sugar, flour and vanilla until well blended. Gently stir in rhubarb and strawberries. Pour into pastry shell.
For topping, in a small bowl, combine the flour, brown sugar and oats; cut in butter until crumbly. Sprinkle over the top of the filled pie.
Bake at 400°F for 10 minutes. Reduce heat to 350°F, bake for 35 minutes longer or until crust is golden brown and filling is bubbly.
Cool on a wire rack, then dig in!