Lightly grease a 9×13-inch baking dish with butter or vegetable oil.
Cut the French bread into bite-sized cubes and fill the baking dish about three-quarters full.
In a large mixing bowl, whisk together the eggs, milk, vanilla extract, cinnamon, and brown sugar until smooth.
Pour the custard mixture evenly over the bread cubes, making sure they are well coated.
Scatter the sliced strawberries and blueberries evenly over the top.
Cover the dish tightly with plastic wrap and refrigerate for at least 4 hours or overnight. This resting time helps create the perfect Berry French Toast Bake texture.
Preheat the oven to 350°F (175°C).
Remove the casserole from the refrigerator and bake for 35–50 minutes until the center is set and the top is lightly golden.
Dust with powdered sugar before serving.
Serve warm with generous maple syrup.