Strawberry Rhubarb Crunch Pie

I'm going to be making this for church potluck on Sunday...hopefully it turns out well!

Pastry Crust Makes Two Crusts
1 1/2 cups gluten-free flour
1/3 cup cornstarch
1/3 cup sugar
1 teaspoon xanthan gum (Bob's Red Mills)
8 tablespoons butter, cut into pieces
1 large egg
1 tablespoon gluten-free vanilla extract

Filling
3 cups chopped fresh rhubarb (or frozen, thawed)
1 cup sliced fresh strawberries (or frozen,thawed)
1 cup sugar
3 tablespoons gluten free flour
1/2 teaspoon lemon juice

Topping
1/3 cup flour
1/3 cup gluten free oats
1/2 cup brown sugar, packed
1/2 teaspoon ground cinnamon
1/3 cup hard margarine or butter
Directions
***Pastry Crust Instructions***.
In a mixing bowl stir together the flour and cornstarch.
Then add the sugar, butter, egg and vanilla and mix until combined.
Remove dough from bowl and knead and fold on a surface dusted with GF flour until well well combined.
Divide into two pieces and wrap in plastic wrap, place one in the refrigerator to chill and place the other in the freezer for later use.
--Go ahead and make Filling while the Pastry Crust is chilling.
--.
After approx 20 minutes, remove dough from refrigerator and roll into a ball.
Using a rolling pin, roll the dough out on a lightly GF floured surface until it is the thickness of a normal pie crust.
Place crust in a 9 inch glass pie dish that is lightly sprayed with non stick spray.
Using a fork poke a bunch of holes in the sides and bottom of the crust.
Combine fruit, sugar, flour and lemon juice and stir well.
Turn into pie shell.
To make topping; combine dry ingredients and cut in margarine until crumbly.
Sprinkle over rhubarb mixture.
Bake at 375 degF for about 50 minutes.