Grease a 9-in x 13-in (23cm x 33cm) baking pan. Preheat oven to 350°F (180°C).
Combine strawberries, sugar, cinnamon, and corn starch in a large bowl. Mix well and set aside.
In another bowl, combine brown rice flour, tapioca flour, quick rolled oats, brown sugar, and baking powder. Cut in butter with a pastry blender or two butter knives until it resembles coarse breadcrumbs.
Press half the mixture into the prepared pan. Top evenly with strawberry mixture.
Sprinkle the remaining half of flour mixture on the top and press down with the back of a spoon.
Bake until light brown, about 50 minutes. Allow it to cool completely before cutting into squares.