Grease a 9x13-inch baking dish with vegetable oil or butter.
Cut French bread into cubes and fill the baking dish about three-quarters full.
In a mixing bowl, whisk together eggs, milk, vanilla extract, cinnamon, and brown sugar until well combined.
Pour the egg mixture evenly over the bread cubes in the baking dish.
Top with sliced strawberries and blueberries.
Cover the casserole with plastic wrap and refrigerate for at least four hours or overnight.
Preheat the oven to 350°F (175°C).
Bake the casserole uncovered for 35 to 50 minutes, checking at 35 minutes for doneness.
Sprinkle the top with powdered sugar before serving.
Serve warm with maple syrup and enjoy.