In a medium bowl mix brown sugar, gluten-free oats, and ¾ cup gluten-free flour. Use a fork or pastry cutter to cut in the butter, until the topping is crumbly. Refrigerate the topping while you prepare the filling.
In a medium bowl, mix strawberries, rhubarb, white sugar, ¼ cup gluten-free flour, and optional orange zest. Place filling in a 9" pie pan or 8x8 baking dish.
Scoop and gently spread the oat topping into an even layer over the strawberry rhubarb mixture.
Bake for about 45-50 minutes until the topping is golden brown and the filling is bubbly. Cool for 20 minutes before serving topped with vanilla ice cream.
Notes
Kids in the kitchen option: In step 2, place all topping ingredients in a 1-gallon silicone or Ziploc bag, close bag, and allow children to squeeze the topping ingredients together by hand.