Lightly grease a 9×13-inch baking dish with butter or cooking spray.
Cut the French bread into bite-sized cubes and spread evenly in the prepared baking dish until it's about three-quarters full.
In a large mixing bowl, whisk together the eggs, milk, vanilla extract, cinnamon, and brown sugar until smooth.
Slowly pour the custard mixture evenly over the bread cubes, making sure all the bread is coated.
Scatter the sliced strawberries and blueberries evenly over the top.
Cover the baking dish tightly with plastic wrap and refrigerate for at least 4 hours or overnight.
Preheat the oven to 350°F (175°C).
Remove the casserole from the refrigerator while the oven heats.
Bake for 35–50 minutes, or until the center is set and the top is golden brown.
Allow the Berry French Toast Casserole to cool for about 5 minutes.
Dust generously with powdered sugar.
Serve warm with plenty of maple syrup.