Grease a 9"x13" baking dish with vegetable oil or butter. Cut the French bread into cubes and fill the dish until about three-quarters full.

In a separate large bowl, whisk together the eggs, milk, vanilla extract, ground cinnamon, and brown sugar until blended. Pour this egg mixture over the bread cubes in the casserole dish.

Slice the strawberries and scatter them along with the blueberries over the top of the casserole.

Cover the dish tightly with plastic wrap and place it in the refrigerator for at least four hours or overnight for best results.

When ready to bake, preheat the oven to 350°F. Remove the wrap from the casserole and bake for 35-50 minutes. Start checking for doneness at the 35-minute mark; if it's not set, continue baking until fully cooked.

Once out of the oven, dust with powdered sugar. Serve warm with a generous drizzle of maple syrup.