Preheat your oven to 350° F and grease a 9x13 baking dish with gluten-free cooking spray.
Add the toasted bread to the pan and sprinkle the blueberries over the top of the bread.
In a large measuring cup or medium-sized bowl whisk together the eggs, milk, granulated sugar, lemon juice, lemon peel, and pure vanilla extract until well blended. Pour the egg mixture evenly over the bread pieces.
In a small bowl add the gluten-free flour, brown sugar, salt, and melted butter and mix until crumbly. Sprinkle the crumb mixture evenly over the top of the casserole.
Cover or store left-overs in an air-tight container and refrigerate. Reheat the casserole in the microwave or oven. This breakfast casserole is even good cold. (I prefer this casserole cold)