Cut the brioche or challah bread into 1-inch cubes. If the bread is fresh, lightly toast it in a 300°F oven for 8-10 minutes to help it absorb the custard better.

In a large mixing bowl, whisk together eggs, whole milk, heavy cream, sugar, vanilla extract, cinnamon, nutmeg, lemon zest, and lemon juice until fully combined.

Grease a 9×13-inch baking dish. Layer half of the bread cubes in the dish, followed by half of the blueberries. Repeat with the remaining bread and blueberries. Pour the custard mixture evenly over the bread, ensuring all pieces are coated.

Gently press the bread down with a spatula to help it absorb the custard. Cover with plastic wrap and refrigerate overnight, or let it sit for at least 30 minutes if baking the same day.

Preheat the oven to 350°F (175°C). Cover the casserole with foil and bake for 30 minutes. Remove the foil and bake for another 15-20 minutes until the top is golden brown and the center is set.

Let the casserole rest for 5-10 minutes before slicing. Dust with powdered sugar and serve with maple syrup or whipped cream.