Prepare the Casserole Base:

1. Cube the Bread: Begin by cutting your loaf of French bread into 1-inch cubes. If the bread is fresh, leave it out for a few hours or overnight to slightly dry out. Stale bread works perfectly, as it soaks up the egg mixture better.
2. Arrange the Bread and Blueberries: Grease a 9×13-inch baking dish with butter or non-stick spray. Arrange the bread cubes in the dish, then evenly distribute the blueberries over the top.

Prepare the Custard:

3. Mix the Custard Ingredients: In a large bowl, whisk together the eggs, milk, heavy cream (if using), granulated sugar, vanilla extract, cinnamon, nutmeg, and salt. Whisk until the mixture is smooth and all ingredients are well combined.
4. Pour Over the Bread: Slowly pour the custard mixture evenly over the bread cubes and blueberries. Use a spatula or spoon to gently press down the bread, ensuring that all the cubes are soaked with the custard.

Let it Soak:

5. Refrigerate Overnight: Cover the dish with plastic wrap or aluminum foil and refrigerate overnight (or for at least 6-8 hours). This allows the bread to absorb the custard and results in a moist, custardy texture when baked.

Bake the Casserole:

6. Preheat the Oven: The next morning (or when ready to bake), preheat your oven to 350°F (175°C).
7. Bake the Casserole: Remove the plastic wrap or foil, then drizzle melted butter over the top of the casserole. Bake uncovered for 45-50 minutes, or until the top is golden brown and the casserole is set in the middle. If needed, cover the casserole with foil for the last 10 minutes of baking to prevent over-browning.
8. Test for Doneness: A knife inserted into the center should come out clean, or with a few moist crumbs (not wet batter). If it’s still too jiggly, bake for another 5-10 minutes.

Serve:

9. Dust with Powdered Sugar: Once the casserole is done, let it cool for about 5 minutes. Dust the top with powdered sugar and serve with maple syrup, whipped cream, or additional fresh blueberries if desired.