Preheat the oven to 350°F (175°C) and grease a 9×13-inch baking dish with butter or cooking spray.
Arrange the cubed French bread evenly in the prepared baking dish. Sprinkle the blueberries over the bread cubes.
In a large mixing bowl, whisk together the eggs, milk, heavy cream, maple syrup, vanilla extract, cinnamon, nutmeg, and salt until well combined.
Pour the custard mixture evenly over the bread and blueberries, making sure all the bread is soaked. Press the bread down gently to ensure it absorbs the custard.
Cover the dish with plastic wrap and refrigerate for at least 2 hours, or overnight, to allow the bread to soak up the custard.
When ready to bake, remove the casserole from the fridge and let it sit at room temperature for 10-15 minutes.
Drizzle the melted butter over the top of the casserole and bake for 45-50 minutes, or until the top is golden and the casserole is set.
Let the casserole cool slightly before serving. Optionally, dust with powdered sugar and serve with extra maple syrup on the side.