In a large bowl, whisk together gluten-free flour blend, gluten-free certified oats, brown sugar, cinnamon, salt, and baking powder.
Stir in melted dairy-free butter and vanilla extract.
Reserve 1/2 cup of the crust for the topping. Press the remaining crust into a 8x8-inch baking dish lined with parchment paper. Make sure the parchment paper hangs over the edges of the pan so it's easy to remove the bars after baking.
In a medium bowl, mix together blueberries, granulated sugar, lemon zest, lemon juice, cornstarch, and salt.
Pour the blueberry filling evenly over the crust.
Sprinkle the reserved topping over the blueberry filling.
Bake at 375F for 45-50 min, until the blueberries begin to burst and the crust is a golden brown. Let sit for at least an hour, until cooled before adding glaze.
In a small bowl, mix together the powdered sugar, lemon juice and lemon zest until smooth. If your glaze is too thick, you can add a tablespoon of non-dairy milk, water or additional lemon juice. If it is too thin, add more powdered sugar. Drizzle the glaze over the bars. Allow the glaze to set for 30 minutes.
Using the parchment, remove the bars from the baking dish to a cutting board. Cut into squares to serve.