In a large bowl, beat the egg, sugar, flour and vanilla together until well blended. Fold in the rhubarb and strawberries and pour into your pie shell.
In a small bowl mix together the flour, brown sugar and oats and butter until the ingredients clump together a bit. Sprinkle the topping mix evenly over the filling.
Bake for 30- 35 minutes until the crust is golden brown and the strawberry rhubarb filling is bubbling.